Return to player (RTP)
Return to player ("taux de retour"): the share of all bets a game is built to pay back over a huge number of rounds.
RTP, return to player (taux de retour au joueur), is a long-run average. A slot at 96% RTP is designed to give back 96 $ for every 100 $ bet, across millions of spins. For your session tonight, it tells you basically nothing.
RTP and house edge
They're two sides of the same coin: 96% RTP equals a 4% house edge. Higher RTP, smaller expected loss over time.
Same slot, different RTP
Lots of studios release one game in a few RTP versions, like 96%, 94% and 92%, and let each casino pick. You can't tell by looking. Open the info or help screen inside the game, at the casino you're actually playing at. The studio's website won't tell you which version you got.
Real-life example
Bet 1 000 $ in total on a 96% version and you should expect to lose 40 $. Put that same 1 000 $ through the 92% version and it's 80 $. Same game, same fun, twice the cost.
Provably fair originals
For games like dice or limbo, RTP comes straight out of the payout formula. You can calculate it exactly from the published scheme.