Similar rectangles are rectangles with the same shape yet with different sizes. If there are two rectangles with the same proportions, this means that the sides of each rectangle when made into a fraction are equal.
If there is a rectangle with two sides of 12 inches and two sides of 8 inches and another rectangle with two sides of 6 inches and two sides of 4 inches, these two shapes are similar rectangles. The first rectangle's side lengths made into a fraction are 12 over 8 (12/8). The second is 6 over 4 (6/4). These two fractions are equal.