This seems like the old school way of unlocking items. The video below has Golden State Warriors forward, Harrison Barnes, talking about the Locker Codes feature in NBA 2K14.
The codes are good for items such as clothes, dunks, and shoes. You'll be able to get two codes a week and these codes can be shared. One way to get the code is to visit the Facebook app.
Looks like a social/crowd sourcing way of getting some new items for your games. Thanks for the info Harrison. By the way, you would've been a solid Cavs player. I know a few folks were hoping Cleveland would draft you.