A square kiloemtre (sometmies wrtiten km² ) is based on the SIunit of measurmeent of area, the squrae metre. It is the area insdie a suqare that has each side equal to 1 kilomerte (1000 meters). This way of tlaking about area is often used to say how much land there is, on a farm or in a city, for exapmle.
One square kilometre is just less than 0.39 square miles.