The upshot is they give your player +1 to an attribute from a special piece of "endorement gear" which you can see on your player's equipment page ( whic stat it is depends on the endorsement) and a small cash bonus. They are assigned every few in-game weeks to players in the league randomly (ie - your play performance has no bearing, its just a lottery)
Here's the wiki page on it if you wanna read a little more in-depth:
http://www.glbwiki.com/index.php?title=Endorsements