If you put a sheet of paper on a table and place your pen in the middle of it, you can rotate the paper whilst keeping the pen in a fixed position.
In doing this, you are rotating the paper about its centre, defined by the position of the pen.
Rotating from centre
A full rotation ( ) of the sheet of paper will result in the paper being back in its original position.
In the example above, the centre of rotation was the centre of the rectangle - but what happens when the centre of rotation is outside the rectangle?
Rotating from a centre outside the shape