means Gambling machines (or their digital cousins) where you spin reels hoping symbols line up to pay out — also, more plainly, narrow openings or assigned positions into which things fit.

from From "slot," a Middle English word for the hollow at the base of the throat, borrowed from Old French "esclot." It came to mean any narrow groove or slit — including the coin-receiving opening on a gambling machine, which is why these games earned the name "slot machines" (and, in Britain, "fruit machines" for their cherries and lemons). The sense of an allotted "slot" in a schedule or lineup is a modern extension of the same fit-it-in idea.