is there somewhere with an ELI5 for this ? I'm a WOW / OW player and all this light level stuff etc is really confusing.
It makes a lot more sense when you play the game.
Assuming it's like D1 my rough guide, no doubt LackyT's will be equivalent of a Prima guide!
You start at level 1 and as you gain experience you level up, nothing different here.
The level cap is currently set at 20, once you reach that experience no longer levels you up instead you get a 'package' in D1 it was 5 motes of light, in D2 it seems you get a loot box.
levels 1-20 you don't need to think about light, it serves no benefit. Once you hit level 20 you'll be able to equip all armour, including legendary (purple) and exotic (yellow). This is when you start thinking of 'light'.
As far as I'm aware 'light' is now 'Power'. I'm not sure of this new cap but for D1 it was 400. This rating was worked out on an average of your gear/equipment. So to be the highest 'level' in Destiny you need to have all of your armour (helmet, gauntlets, chest, legs) class item (hunter cloak, titan cloth and warlock armband), Ghost and weapons at a light level of 400.
in short - pre level 20 only experience can make you 'stronger'. Post level 20 only gear/equipment can make you stronger (by raising your light/power levels)
I've probably confused you even more!