A square kilometer (sometimes written km²) is based on the SI unit of measurement of area, the square meter. It is the area inside a square that has each side equal to 1 kilometer (1000 meters). This way of talking about area is often used to say how much land there is, on a farm or in a city, for example.
One square kilometer is just less than 0.39 square miles.
Square kilometre Facts for Kids. Kiddle Encyclopedia.