It is possible for a square to be a rectangle, but it is impossible for a rectangle to be a square. In order to be a square, the shape must contain four straight sides with 90-degree angles that are all the same length.
A rectangle has four straight sides with four right angles, but the sides can be different lengths. The opposite sides of a rectangle must be parallel with one another and feature equal lengths. This means the shape can be tall and skinny or short and fat. The sides on a square must all be parallel in comparison with one another.