The net of a 3D figure is the 2D surface area all laid out flat. One example would be that you have a box and you unwrapped the wrapping paper, placing it all flat on the table. By "placing it flat", I mean there are no portions that overlap. A 2D net is useful to help see that surface area is really just area. Specifically, surface area is the total area of each face of the 3D polyhedron.