A preschool is a school for kids too young for kindergarten. In preschool, your teacher will still help you tie your shoes and stick your straw into your juice box.
You can use the word preschool for the actual building where three year olds attend school, or for the program there. It's also an adjective for talking about preschool teachers, preschool students, or preschool funding in your town. Preschool has been used since the late 1800s, and it's also commonly called pre-K or nursery school.