Seems Straightforward Enough (Destiny)

by DaDerga, Baile Átha Cliath, Friday, August 01, 2014, 02:59 (3557 days ago) @ kapowaz

You can't grow your characters beyond level 20 by conventional means - collecting experience points. Once you reach 20 you'll see XP collected into Motes of Light instead.
It's these Motes of Light which you can then craft into special armour - courtesy of the Speaker character in the game's Tower social hub.
This armour will boost your character stats higher (the highest shown so far is level 29), although the character menu itself will still show 20 as your upper limit.
These weapons have a light statistic - what we don't know is how these light numbers translate into a character level.

...Once someone bothers to articulate it properly. It's the bolded part that you'd also like to have explained and I can understand why, if you are that way inclined, the evasiveness on this can be irritating somewhat.

Brief article here.