What does the idiom 'rang a bell' mean?

Idiomatically, to ring a bell is to remind someone of something. Here is an example. She was wearing a dress with purple flowers on it; does that ring a bell? Oh yes, I remember her now.

It means that you vaguely remember something, but not the exact details. For example, if someone mentioned a person's name and you remembered that you had heard that name before, but you didn't remember the person, you would say "Her name rings a bell." Also "to ring someone's bell" means to hit them in the head, knock them out - alluding to boxing.
If something "rang a bell", it means that it sounded familiar.