Commendations are awarded by performing certain actions example 'Assistant' - earn an assist in multi player matchmaking. I don't think these unlock anything? Not sure? but you have a few rank levels Iron, Bronze, Silver, Gold & Onyx rank. Currently I have 2 Iron in multi player matchmaking and 7 Iron ranks in fire fight. Hopefully I should be able to get some of these up to Bronze level by next week at least in fire fight anyway.