The RTP is the percentage of bets that a game returns to players over time. For example, if a game has an RTP of 92% it means that the game will return $0.92 of every $1 bet. However, this does not mean that it will return $0.92 of your $1 bet every spin, you could win nothing or you could win your $0.92 plus the $0.92 from many other spins. Nonetheless, by looking for games with higher RTPs, you can reduce the chance of you losing all of your money.
Volatility refers to the amount of risk involved in a game. A high volatility slot means a higher risk, it offers larger payouts but the payouts will be fewer and farther between. A low volatility slot is the opposite, it will payout small amounts on a very regular basis.