A slot machine is so-called because there is a slot that takes your bets in return for allowing a spin of three or more reels. The idea is to try and find matching combinations of icons on a payline across those reels. The games can feature just one line to bet on or many more – sometimes 50 lines or more. Other slot games feature ways to win (1,024 ways to win for example, as the maximum number of combinations given the format of the machine), or cluster pays (where a cluster of symbols must be attained to win a prize).
Every slot machine is designed to pay the house (or casino) an edge. This means for every dollar that is wagered on the machine, the house will get a few cents. This could be two or three cents, for instance. The house edge is given as a percentage, i.e. 2.5% on a slot machine that offers the player an RTP of 97.5%. Add the two together and you get 100%. This is how to work out the house edge on any machine you play.
The odds of getting certain combinations are calculated, and the prizes are then worked out to be slightly lower than those odds. Incidentally, you should get better odds at online casinos compared with bricks and mortar casinos. The same game, for example, might have an RTP of 95% in a real casino but offer 98% at an online casino. The reason is that the overheads of an online operation are far lower. Always look for the best RTPs you can get, accept the house will always have an edge, and remember great prizes can be won in the shortest of gaming sessions if the player gets lucky. You just cannot rely on it though.