A. RTP % is the percentage of money that is paid back to players - in other words, the amount of winnings divided by the total amount of bets. For example, if an online casino has a 97% payout percentage, that means that for every $100 wagered, $97 will be paid out in winnings.
This number can fluctuate quite a bit from one casino to another - and from one game to another within a given casino - but in general, the higher the better. A high payout percentage means that players are more likely to walk away with winnings, which is obviously very attractive to most gamblers.