Similar is a term used in math when discussing geometric figures or shapes, and it means that both figures' corresponding sides are proportional, but the figures themselves are two different sizes. So one is basically a larger version of the other.
Similar figures make sense when thought of as resizing. If one takes a shape and simply uses a tool to enlarge it, then the new bigger shape and the original smaller shape are similar. Each of the shapes' corresponding sides are proportional to the other. On the other hand, if two figures or shapes are the same size then they are congruent.