A cube is a square box (or block) shape:
The faces of a cube are all square.
Here is a rotating view of a cube:
A cube has 6 faces - the top, bottom, and 4 sides.
It has 8 corners (vertices).
It has 12 edges.
Here is a cube of side $a$:
The width, height and length of the cube are all equal to $a$.
This means that the volume is:
$$ volume = a \times a \times a = a^3 $$
The volume is $a$ cubed (this is where the term cubed comes from).
Each of the 6 faces of the cube is a square of size $a$ by $a$.