The net of a 3-D solid is the shape that needs to be cut out of cardboard or paper to fold into the given shape.
Example
This diagram shows how the net of a cube folds into the cube itself.
Remember that each cube will have the following...
A BASE, a TOP, a FRONT, a BACK, a LEFT and a RIGHT FACE
These can be tricky to spot, so it's a good idea to have a piece of paper in front of you for these questions so that you can try folding or drawing the nets to help you to picture what's going on.