To understand something is to comprehend or get it. The more we learn, the more we understand.
The goal of most education is to help students understand how the world works: history, math, English, music, science, and art are all complicated subjects that you need to practice and think about before you can really understand them. Understanding requires knowledge and thought. Another kind of understanding is like sympathy. For example, you might not approve of stealing, but you could understand why a guy would steal to feed his family.