RTP is short for Return to Player Percentage. It’s also the expected amount of money the game pays out over a certain period. If the slot has an RTP of 97% it pays back 97 cents for every dollar you put into it.
RTP is just an indication of what could be. Don’t make the mistake of picking a game just because it has a high RTP. Some games have very high RTPs yet they offer lousy payouts.