The return to player (or RTP from now on) is exactly what is says on the tin, it is the amount of money as a % returned to the player for every £1 they put in (wager). You will sometimes see this expressed in its opposite form, that of house edge, which is the amount as a % of every £1 wagered that the casino will keep. If a game has a RTP of 97% it will have a house edge of 3%. They are the two ways of expressing how profitable a game is to the Casino and how unprofitable to the player.
To work out this figure is not always easy when games do not have defined probabilities. In a game such as Roulette it is relatively easy to work out, the odds and payouts are clearly visible and 36/37 (because there is one Green 0) gives 0.973 as the RTP. If you took out the Green 0 and just had 36 numbers and a 1 in 36 chance of landing on each number then it would be 1, in effect 100%, the players would never lose money in the long run.
Slots games are much harder to calculate the RTP as it is not clear what probabilities there are for any given win to occur or always what any given win (if a Bonus Feature round or Free Spins) will award. The programmers set the slots to run with complex algorithms which ensure that, over the long run, the amount of money taken in by the game and the amount paid out as winnings equal this percentage, but it is in reality very difficult (if not impossible) to know for sure and in the end you just have to trust the Casino auditors.
To make a profit from slots, or at least to have the best chance of being in profit at some point, you want to select a game with as high a % RTP as possible, and also one with as much variance as possible. This will ensure that during the swings of the game taking money in and paying money out you have the best chance that at some point an upswing will take you into a profit situation. The worse thing you can do is play at a low RTP slot with low variance which will just slowly but surely suck your money away with no significant boost to give you an opportunity to “Quit whilst you are ahead”.