Self-Similarity
Self-similarity is when a shape's smaller parts look just like the whole shape.
Think of it this way
A head of broccoli shows self-similarity perfectly. Each big floret looks like a tiny copy of the whole head. Break off a smaller piece, and it still looks like the full broccoli. The same shape repeats at every size.
Explaining self-similarity by grade level
Cut a paper hexagon into smaller hexagons. Each small piece looks like the big one. Cut those pieces even smaller. They still look the same.