What did Mr. Auld forbid Mrs. Auld to do? Why?

Mr. Auld forbids his wife from teaching Frederick Douglass how to read. Mrs. Auld is a very kind, considerate woman. She doesn't like it when Frederick adopts a posture of cringing servility towards her, but it's all he knows. Mrs. Auld has never owned a slave before, so she has no experience of how most owners treat their slaves. For Frederick and Mrs. Auld, this is a whole new experience for both of them.
When Mr. Auld catches his wife teaching Frederick the alphabet, he's absolutely furious. Teaching a slave to read isn't just wrong; it's against the law. If slaves are taught how to read they'll start getting ideas above their station. Then they're liable to become resentful about their condition as slaves. Mr. Auld genuinely believes that teaching slaves to read will seriously undermine the foundations of society.
Mr. Auld's intervention backfires terribly. It simply makes Frederick all the more determined to learn how to read, as he now realizes that lack of education is used by white people to keep slaves in a state of ignorance and subjection.