RTP stands for "Return to Player". In simpler terms, it refers to the payout rate of a gaming machine.
The RTP indicates the percentage of wagers that a game returns to players over the long term.
The average displayed RTP pertains to the machine itself rather than individual players: a machine showing an RTP of 96% does not guarantee that you will recover €96 from every €100 wagered.
This should be viewed as an average calculated across all players over millions of spins.
For instance, if X players each wager €100, then on average, €96 will be returned among these players: some will receive more, while others will receive less.
You can find the RTP percentage for each machine by consulting the Information provided for each one.
May luck be with you!