God of War's debuffs system is not well-explained. This guide seeks to help you understand what your light or shock arrows do to enemies, what immolation and frostburn do, and how poison impacts you on regular and Give Me God of War difficulty. I went through the whole game without really noticing these icons, but they can be useful tactically. If an enemy is just a shot away from being electrified I want to know and exploit that! This is more important in GMGoW when enemies may enrage/level up and you need to focus them down one at a time.
Above health bars - both Kratos and enemies' - small icons appear to denote that a debuff has begun to be applied. As more of this effect is felt, the icon changes to represent that it has fully taken effect and will take some time to clear. Once it's completely full and the timer around the edge (pictured below) begins to count down the enemy will be immune to further debuffs until it's cleared. Kratos and Atreus can inflict a few of these toward enemies - namely shock, permafrost, immolation, and weaken. Blind and poison status effects can only be applied from enemies to Kratos. Atreus seems to be immune to debuffs.
Enemies and Kratos hit by ice or fire for long enough begin to suffer effects. Kratos can only charge his weapon up with the Permafrost and Glacial Permafrost Skills. At first it doesn't do anything, but momentum in combat will make his weapon begin to glow. At that stage just a few hits can begin to apply permafrost to his enemies. Permafrost, when fully applied, makes Kratos and enemies slower. Immolation on the other hand, does direct HP damage. Aside from the icons, it's easy to tell when either is applied as the enemy will either appear very cold or the victim will be burning.
After 5-8+ consecutive hits without being touched by an enemy, Kratos weapons are gradually powered up, and will apply more of the debuff on each hit (so it takes less time to apply permafrost on a newly targeted enemy). In addition to this, the attacks now deal extra magical damage on each hit in addition to the normal damage that is determined by Strength. The additional magical damage is boosted instead by the Runic stat - every single hit, no matter what special you perform (maybe not applied to light/heavy runic attacks). Maintaining your momentum in combat is key. Go long enough without attacking and the glow will disappear along with the bonus damage. You're required to have certain skills to get this bonus - those that say the weapon will build up permafrost. Certain enchantments can increase the elemental damage Kratos deals or increase the rate that the weapon charges up. This effect is actually kind of rare and only found on a couple of enhancements. They can be a key part of a build.
Atreus' debuffs are interesting and highly useful. Something worth noting here is that for Runic Summons, the type of arrow you have selected does matter in certain cases. The Elk and Falcons both have high runic stat, while murder of crows and wolves don't. The more runic, the more of that arrow's specific type of debuff will be applied. Some enemies are more resistant to debuffs so hitting them with a summon that has runic and having a volley of arrows ready can help you to apply the debuff. This is the sole purpose of that stat, as far as I can tell.
Shock arrows are amazing for clearing crowds, because when upgraded they will apply a chain lightning effect. They're actually less effective on single targets unless it's a trash mob (you won't be able to lock down a Traveler or other strong enemy) because you'll lack the chain lightning. The lightning spreading to other enemies begins to build the debuff meter, while the original target suffers along with any that are chained and may be hit a second time by hits off his buddies. It can be effective to fire at different areas of the crowd because each shot can bounce a couple of times and you'll get quite the light show. When the meter is full, enemies are shocked relentlessly and frozen in place. It's not a stun (so no R3) but you will have no trouble targeting line-type Heavy Runics or using a well-placed Executioner's Cleave.
Light arrows confused me a bit at first. They do heavier stun damage while shock arrows do very little comparatively. With this bonus, what more could they add? Much more! Light arrows apply a debuff along with the stun damage that lasts about 10 seconds. You've got to fill the meter like with other debuffs, but once that is done the weaken effect is applied. Instead of chaining, the light arrows have a small aoe near the target that can begin the process of building up the debuff. Weaken is more noticeable on higher leveled enemies or those that match you and are harder such as Travelers and Revenants. The weaken debuff effectively lowers an enemy's level, making them deal less damage (I took 33% less damage in playtesting) and suffer more damage. What is more, it lowers their resistance to stun while the light arrows themselves help to build up the meter. Follow up with some bare-handed attacks to get it to full and have fun smashing face! When an enemy was 2+ levels lower it didn't do anything noticeable, but it did when they were equal or higher.
Used by both Revenants and the annoying burrowing sabretoothed-gophers that everyone hates. Poison builds up like other debuffs and can deal damage over time. In Give Me God of War it will also lower your effective level, while enemies can also level up. It's very nasty in that way. Evidently health pickups may help you to cure it if you're poisoned. A word of advice for dealing with both of these - use a full volley of Atreus' arrows while Kratos is on approach, in between waves of poison. When you get close enough that the Revenant is still there but not teleporting (or burrowing, in the case of gopher beasts) you can unleash with Atreus while Kratos steps in. Even if you don't manage to stun/shock the enemy the tiny interrupt from Atreus' attacks will result in an opening for Kratos to do a combo or Executioner's Cleave. It's also fun and very satisfying to go in with some bare-handed attacks and stun grab (R3).
Dark-elves specifically use a lot of blinding attacks, and they impact you right away unlike others. You can try to avoid this by blocking or simply push down on the d-pad to look the other way. If you're blind, use Atreus to help you figure out where enemies are. Dark-elves like to move around a lot so it can be highly frustrating. Blindness heals up on its own after about 10 seconds. Daze is another debuff mentioned on an enchantment that increases Kratos' resistance to debuffs, but I cannot recall what enemy dishes it!
God of War Tips - My Intro Page with loads of advice.
God of War Stats: What Luck, Cooldown, Strength, Defense and Runic do for Kratos
Using Stats to Make a Build
Atreus in Early to Mid Game - Making use of Atreus and upgrading his abilities.
Niflheim (End Game Content) - Increase Mist Resistance.