Condescending means being kind in a way that makes clear you think you are superior. The compliment that is really a pat on the head.
The tone is what gives it away, since a condescending explanation is one that assumes you would not otherwise follow. It takes to. The word once meant simply stooping to somebody's level, and the disapproval has crept in over centuries, which is why older writing uses it as a virtue. Patronising is the near synonym in British English, and both are among the most stinging accusations you can make about manner.
1adjectivecondescendingBehaving as though you are superior while appearing kind.
“He has a condescending way of explaining things.”
synonyms: patronising