"Rotation" means turning around a center:
The distance from the center to any point on the shape stays the same.
Every point makes a circle around the center:
|Here a triangle is rotated around
the point marked with a "+"
Try It Yourself
Here you can drag the pin and try different shapes:
And here you can choose an angle and see how to rotate different shapes point-by-point. Try and follow what happens each time: