A lot is a plot of land. When you buy a house, you also buy the lot on which it sits.
If there's an empty lot in your neighborhood, it means there's a space covered with grass or pavement that's large enough for a house or building to fit there. Another kind of lot is a set or group of items. At an auction or estate sale, you might find a lot of antique silver spoons for sale, or a lot of comic books — in either case, the lot is a collection.