Casinos are using the RTP term to describe the percentage of all money paid into the game that is paid out in winnings over some time. 

Example:

If the RTP for a game is 80% and you play on €1 bet, you should expect €80 in 100 spins.
If the RTP for a game is 95% and you play on €1 bet, you should expect €95 in 100 spins.

Because of RTP, people prefer online casino to a land-based casino. An online casino usually has 95% RTP while the land-based casino has 70-90% RTP. You should always avoid slots with less than 95 % RTP.

With a little help from us, you can find high RTP slots, but don't forget the main point of slots. They should not be anything but a hobby. We recommend that you always try a demo version of a slot and then play for real money. Maybe you will like the 95% slot more than 96%slot because of graphics, features in the slot, more entertaining free spins, bigger max payout etc.
Remember RTP percentage is always theoretical and shouldn't be the rule that you will win less on lower RTP slot.