You will start finding armour with light pretty much everywhere. The more light, the higher your level. The amount of light on a piece of armour is displayed with the amount of discipline etc... As you get to higher levels you will start unlocking things like raids and weekly strikes. These will generally drop weapons/armour/upgrade materials. When you upgrade the defence of a piece of armour you increase the light level that it carries. Light only appears on rare/legendary/exotic. I hope this helped and reply if you need further clarification :)