What does it mean to understand?

Letters for Future You Vol.1
8th Letter. What does it mean to understand?
In this letter, I want to talk about what it means to know and understand the purpose of learning. In my case, I learned it gradually while learning math. So, I have no choice but to talk about math. It may be boring, but I still think it's an important story to share.
The Book link
www.amazon.com/dp/B0BZN497YB