AI - Artificial Intelligence (or just the computer opponents you can play against). So play any match online (after logging into your account) and finish it. You can play against those computer opponents or human players (doesn't matter). After the match, they probably should be unlocked.
Edited: Forgot to tell you, that you'd better use google for such simple questions (google "AI"). There were even some topics on forums about this same problem, but it's more difficult to find.