A net is a two-dimensional (2D) diagram that shows the sides of a three-dimensional (3D) shape when it is 'unfolded'.
If you cut out a net, you can fold it up to make a model of the 3D shape.
Example 1
A cube has been "unfolded" to create a net with 6 regular faces (each face is a square).
There are 11 nets that make up a cube (shown below).
Example 2
A cuboid has been "unfolded" to create a net.