A slot is a narrow opening in a machine or container, for example a hole that you put coins in to make a machine work. He dropped a coin into the slot and dialled the number. If you slot something into something else, or if it slots into it, you put it into a space where it fits. He was slotting a CD into a CD player.
We call it The Slot Method. Using this method, when asked to find a probability of a compound event A, we will find the total number of outcomes, n(S), and the total number of successful outcomes n(A) and use slots for each of the number of events (either independent or dependent) that make up the compound event.