A rectangle is a four-sided shape, and all the angles are right angles. If you want to figure out if a shape is a rectangle, you have two tests. Is it four-sided? And, are all the angles 90 degrees? If both answers are yes, then you're looking at a rectangle.
Rectangles have a few special features. First, the opposite sides are parallel. Second, the opposite sides are equal in length. In the rectangle shape, if we know the side, AB is parallel to side CD, and BC is parallel to AD. Also, if we know AB is six, then so is CD. If BC is four, then so is AD.