The D stands for "dimensions ."

1 D Here is a line. This has got one dimension (length)  

2 D Here is a rectangle . It has got It has got two dimensions, length and breadth (width) . So it is 2D.

All these shapes are 2D: Squares, circles, triangles, parallelograms and all the polygons (hexagon, pentagon etc)

They have area but no depth.

3 D So all 3D shapes must have depth they are solid. 
All 3D shapes have volume.

The cube has got all sides of equal length ( a 3D version of a square)