Others can explain this in a more informed, thorough way, but just to get you started: All of your gear (primary weapons, special, heavy, ghost, helmet, gloves, chest piece, boots, class item, artifact) contribute to your light level. To reach the current light level max of 400, all pieces of gear must be 400 (for example, if all pieces are 400 except one which is 390, you'll be at 399 light). Your light level contributes to your total ability to do damage, and to be shielded from damage. I don't know the specifics, but there are numbers like- if you are three light levels below an enemy, you'll only do 40% of the damage you would against them if you had the same light level, and you'll take 20% more damage (these numbers are made up for the sake of making the point). So for example when you went into Crota hard mode back in year one as light level 31 for a raid that was light level 32, you were doing less damage and taking more damage. Plus, the bosses were a level higher (I believe Crota was level 33). So right now, people ask what light level they need to be to do the new raid. There's a recommended light level, but you can sometimes get through at lower light, but if you go in at max light you'll find it is much, much easier. For example, I believe the first times I tried Wrath of the Machine (normal mode) I was at 370 light. Going in at 400 light it was quite easy. I don't know if this is specifically what you were asking about, but I hope some of it helps. I remember at some point someone saying the specific light level of a gun didn't matter- it was your total light that determined damage (let's say all your gear is 400 except for a 350 gun). But I later read that this was not true- that the light level of your gun does matter beyond what it contributes to your total. But other people know a lot more about this kind of thing than I do.

It's actually much more simple now. The light level is the only thing that truly exists now. You still have to acquire higher light items to reach max light but it is now done via infusion. Say you obtain a 398 chest piece you don't like while wearing a 387 chest piece you do like, so you then "inspect" the piece of gear you like and in the left hand column there is a node that is labeled "Infuse". Once you open that it will provide you a view/choice of items you have that you can use to make the current piece of gear a higher light level via some materials. That's it. Hope that helps.