What does it mean to be a 3d figure?

Answer 1

3D means three-dimensional. A flat object or figure - anything you can draw on a piece of paper, for example - has 1 or 2 dimensions; anything that also has thickness, and that you CAN'T draw on a piece of paper, is 3D.

Answer 2

It means that the shape occupies three dimensions and so has a volume. The 3 dimensions may be called length, breadth and height but in some cases breadth is called width, height is called depth.