A ‘buy-in’ is what each player pays to enter a tournament. If the buy-in is play-money, a certain number of play money credits are used to sign up. Tournament winners win money credits in return.
A ‘freeroll’ tournament does not require a real money buy-in but pays out a prize pool of real money. These are a sensible way for a beginner to start because there’s no risk of losing any money but a chance to win some in return.
A real money buy-in involves using real money that has been deposited in an account to buy in.