I just bought one but I don't know what it does or how to use it, any. HElp?

Exp lock stop your pokemon from leveling up, so if you want to keep a certain pokemon at a certain level, and want the pokemon in your roster, but afraid that you might levels it up accidentally, you can attached the exp lock to it (just like any other item you attached to a pokemon), and it wont level up if you defeat a pokemon with it.

just a side note: after you attached it to the pokemon, switch another pokemon in the first spot, since there a glitch that makes any item you attached to the first pokemon a exp share (temporary, until you switch spot in your roster or logged out).