What does 'rtp' stands for?

Return to Player (RTP) is the term casinos use to describe the percentage of all the wagered money a VLT or slot machine will pay back to players over time. For example, if you make a hundred $1 bets on a machine on which the RTP is 90%, you might expect to get back about $90 in wins. Of course, both house advantage and RTP are generally calculated over the long term. Almost anything can happen in the short term (see Short Term Volatility versus Long Term Predictability), so you should only ever bet with money you’re prepared to lose.

You may have already noticed that RTP is really just the flip side of house advantage. If a machine has a 20% house advantage, then the average RTP will be 80%, meaning that over time the machine will keep about 20% of all money bet and will return the other 80% to players in wins.
Most online slots are between 93% to 96% RTP. It also differers a lot if you want to play high versatile games or if you rather grind out the games with lesser risks! Games with lower RTP might have a chance for a higher pay out, although this isn't 100% the case for every slot.