I'm going to try my best to explain 90° rotation:
So, you know that if you rotate something 180°, it's completely flipped (think about spinning around half-way).
Or if you spin something 360°, you spin around the whole way and end up in the same spot that you did when you started.
Notice how 90 is actually 1/4 of 360.
So imagine spinning instead of 180, spinning half of that. so you <em>barely</em> rotate. That's exactly what you're doing to this shape here. and if you do it about the origin counterclockwise, the origin is (0,0) so I drew it in Quadrant III, as you can see in my attachment.
You can see that every point has been moved by 90°, I put all of the variables there so you could visualize it better!
I hope this helped, let me know if you have any questions! :)