Quite simply the higher the value the more money a player can expect to receive over time, on average, when they play that game.
Many slot machines have the RTP of the game displayed for players to see, usually on the information or rules page of the game, so it is always something worth checking out.
Generally, in terms of RTP, anything with a figure of 98% or above is considered high, a mid-range or average game would fall into the 95% to 97.99% bracket, whilst anything below 94.99% would be considered low.
The actual RTP of a game is unlikely to be reached in one single session because, as has been mentioned, it is something that is calculated in the long-term. However the basic fact is that, on average, a player is likely to win more often on a game with a 98% RTP than one with a figure of 90%.