Slots usually refers to casino gambling machines known as slot machines. These usually contain 3-5 reels with around 22 different images on the reels. A user inserts money (or casino tokens) and spins the reels, if specific patterns of images appear such as 3 of the same kind the user wins a cash prize.
Slot machines give the appearance that there is an equal chance of stopping at each position on the reel but this is not the case as each position has a weighting that determines the likelihood the reel will stop at that point. Based on the reels and this weighting it is possible to calculate the payout percentage.
More recently the trend has been away from the mechanical spinning reels to digital versions with simulated reels displayed on a screen, and so are almost identical to the online versions.