It's a system that came out with mods during Season of Dawn. Some mods will give you stacks of a buff called Charged with Light while other mods consume the stacks to give you benefits.

There are three type of mods -ones that grant you charged with with light based on the requirements of the mod (taking charge is the easiest way) -ones that consume the charges of light and give an effect of some sort. -utility ones for example, modify the amount of charges with light you can hold. Neutral type Charged with light mods can go on any armor type. Solar affinity CWL mods only go on solar armor and have effects that stack. Arc affinity CWL mods are expensive but offer a secondary effect when a 2nd arc affinity mod is slotted into the same armor piece. Void affinity mods have a stat penalty but the ability provided may be worth the trade off.

Believe it or not but charged with light mods are enabled in pvp, high energy fire for example is really overlooked since its a 20% damage buff until you die or kill sombody. Its definatly worth looking at the mods if you havent yet, they're pretty neat