Learning Quaternions
3blue1brown makes lots of good videos on mathematics. One of those videos is how to visualize and understand quaternions.
Quaternions are a higher dimension system that can be used to describe 2D and 3D rotations. How they work, however, is often much harder to understand and more complex than understanding simple matrix rotations.
They made a very good video on the subject, but it required me to stop a lot and spend time thinking. These are complex concepts and almost more complex to visualize or conceptualize in your mind.
What’s nice is there is a written page that goes over these concept as well at https://eater.net/quaternions. I found this was much easier to digest than a fast-running video.
Also, if you want to play with the visualization in realtime, they even have a super-cool tool that lets you play with Quaternions in 2D, 3D, and 4D: