I've heard it in different forms: "Them that can, do. Them that can't, teach."
"Them as can, do. Them as can't, teach." It is a rather snobbish opinion of teachers, and the teaching profession in general. The idea is that if I have an interest in a field (biology, for example) the highest expression of my interest will be doing biological research. If I can't 'cut it' as a research biologist, I can always fall back on teaching. Those that can, will become biologists. Those that can't become biologists will end up teaching biology. For some, there may be some truth to this. But it is an unfortunate slur on the majority of teachers who enter education enthusiastically and with rigorous training. Never have we (in the US) had a greater need for trained and eager teachers. There are countries where teaching and education are held in the highest esteem. We need to get back to that point. ANSWER A footnote: As a way of accusing colleges of Education, professors of Education, and degrees in Education of being academically inferior, a corollary arose: Those who can't teach teach teachers how to teach.