The Return to Player percentage (RTP) is the percentage of all wagered bets that the slots pay back to you over an extended period of time. For example, with an RTP of 90%, you will receive back on average R90 for R100 worth of bets. A good RTP is approximately 95%. The higher the RTP, the better payouts players can expect on a particular game.
E.g. if a slot machine has an RTP of 95% and you place R1000 in bets on the machine, you can expect the machine to return / pay back R950. In reality, the machine will pay random amounts and it is over a longer period of time that you will see 95% in returns. The deviations of the RTP in short term are what allows players to win at the casino.