The "D" refers to as Dimension. The figures 2D and 3D refer to the work dimensions on any surface or, specifically, the computer workspace. 2D means in two directions along with X and Y-Axis, and these are "Flat Shapes".
Contrarily, a 3D Figure is expended in three dimensions and hence, has depth in the Z-axis. Upon rotation, a 2D turns to a flat shape, whereas a 3D figure offers multi-dimensional visibility.