George Bernard Shaw said. He who can does. He who can't teaches. What does this mean?

This is a slur against teachers. It means that if a person is really good at doing something, then he will spend his time and make his living doing it, rather than teaching other people how to do it. For example, if you can really play the piano, you will be too busy and earn too much money giving performances to want to spend time being a piano teacher.