RTP stands for Return to Player Percentage. RTP is used to demonstrate the likelihood a player has of winning common casino games like slots. RTP is displayed as a percentage.
For example, if a RTP is 97.5% for every £100 wagered the game should payout £97.5 giving the game a margin of 2.5%. The percentage displayed is an average percentage measured over 10,000, 10,000 or more games. Which means the RTP is unlikely to apply to a single or even multiple sessions played by a single player.
All casinos regulated by the UK Gambling Commission are required to clearly display the RTP and there is no minimum Return to Player Percentage requirement.