Some 3D shapes, like cubes and pyramids, can be opened or unfolded along their edges to create a flat shape.
The unfolded shape is called the net of the solid.
Here are some 3D shapes and their nets.
3D shapes and their nets
A net has to be able to fold up into the 3D shape. All of the following shapes are nets of a cube.
Nets of a cube
The shapes below are not nets of cubes. Consider why they wouldn't fold up into a cube?