A square is a flat shape, in one plane, defined by four points at the four corners. A square has four sides all of equal length, and four corners, all right angles (90 degree angles). A square is a kind of rectangle.
To figure out the area of a square, you multiply the length by the width. Because the length and the width are always the same, you could also say that you square the length of any one side of the square (that's why they call it the "square"). To figure out the perimeter of the square, you can add the lengths of all the sides together, or multiply one side by four, because all the sides are the same length. So if your square has sides 5 centimeters long, the area of the square is 25 square centimeters, and the perimeter of the square is 20 centimeters.