There's 17 codes out since E-3 last year, each code unlocks something in the game that has been unannounced. There will be 25 in total, wait for more codes during, and after, this year's E-3.

We still dont know for sure Guardian. It could be anything from getting interesting and rare loot in game or just cosmetic appearances to armor or anything really. Until Bungie says otherwise we just need to check back later hopefully before the release but until that time were told or notified anythings possible.