When you cannot teach someone about something, that means you do not understand it. One way to understand something better is to start teaching it to people, learn from it and improve on it.

Someone (Einstein, Richard Feynman or somebody) put it well,

“If you can’t explain it simply, you don’t understand it well enough.”

Contrary to that idea, to get better at anything we need to repeat it and rinse. So, instead of waiting to fully understand a subject before teaching it, one can start teaching it and the process of teaching someone clarifies things in our own head and improve our understanding of the subject.