A shape has rotational symmetry if you can rotate it round its central point and cause it to occupy exactly the same space more than once.

With an irregular shape it is obvious that there will be no rotational symmetry. But take a regular shape, for example a square: it occupies the same space for every 90° of rotation, in other words four times in a revolution. It is said to have an order of four of rotational symmetry. A rectangle has only an order of two, as it takes a rotation of 180° before the shape occupies exactly the same space again.

A shape with no rotational symmetry can be said to have an order of one, using the same logic.