A rectangle is a flat shape, in one plane. It’s defined by four points at the four corners. A rectangle has two sides of equal length opposite each other, and two other sides of equal length opposite each other, and four corners, all right angles (90 degree angles). A square is a special kind of rectangle. In a rectangle, the two long sides are always parallel line segments, and the two short sides are always parallel line segments.
To figure out the area of a rectangle, you multiply the length by the width. To figure out the perimeter of the rectangle, you can add the lengths of all the sides together. So if your rectangle is 10 centimeters long and 5 centimeters wide, the area of the rectangle is 50 square centimeters, and the perimeter of the rectangle is 30 centimeters.
If you move a rectangle through space, you get a solid form called a rectangular prism. If you divide a rectangle in half diagonally from corner to corner, you get two right triangles. So you can find out how long that diagonal line is using the Pythagorean theorem.