Similar figures are figures that have the same shape, but not necessarily the same size. In the figure below, the square with a side length of 2 is similar to the square with a side length of 1.
For example, you could say that the small square by enlarged by 2 or the big square was shrinked by 2. It is important that all sides are enlarged or shrink by the same amount. In a square, it does not make a difference since all sides are the same. However, take a look at the following figure.
The small triangle is not similar to the big triangle because the sides were not enlarged by the same amount.