There is a particular shape that you can build with three copies of the piece on the left, or with 5 copies of the piece on the right. You may rotate and flip the pieces, but they cannot overlap. What shape can you build?
These shapes, made up of multiple connected squares are called "polyominoes". The polyomino on the left is called a U pentomino. The one on the right is called an I tromino.
If that's too easy, try this one:
The polyomino on the left is called an L pentomino, and the one on the right is called a T tetromino. Note that unlike in Tetris, you are allowed to flip these pieces.