The phrase RTP stands for Return to Player. RTP is used to explain the sum of money, in percentage terms, that a casino game or a casino slot can be expected to pay back to its customers. It is used by both traditional casinos and online casinos.
This is best explained by using a practical example. Let's say for instance that a slot machine has a 90% RTP, if the customer has $100 and it costs $1 a spin to play then they can expect to end up with $90 after 100 spins. That is the Return to Player value for that specific slot machine.
Now note, this is just an average figure based on the life of the machine, and usually calculated over a minimum play of one million spins. The player could get lucky and walk away with more, or they might end up taking home much less than they gambled.
It is also worth remembering that the figure doesn't necessarily indicate how often a player is likely to win, but just the average percentage return. For instance, there could be two games with a value of 95%, the customer might play the first game 99 times and win nothing but on their 100th play win $95. In the second game, they might play 100 times also, and win on 30 occasions, collecting $95 in total. Both games have an RTP of 95%.