A thin opening or groove in something is a slot. You can put letters and postcards through the mail slot at the post office.
A gum ball machine has a slot for inserting a coin, and some front doors have a special slot for mail carriers to put mail in. A screwdriver fits neatly into the slot on top of a screw. Another kind of slot is a space in a schedule or calendar: "I have a half-hour slot on Thursday for our meeting." In the 14th century, a slot was "the hollow above the breastbone."