What does the idiom tip of my tongue mean?

The idiom "on the tip of my tongue" means that you know the answer to a question or the word you're looking for, but you can't quite remember it at the moment. It's as if the answer is just about to come out of your mouth, but it's stuck there, right on the tip of your tongue.

Here's an example:

* "What's the name of that restaurant we went to last week? It's on the tip of my tongue!"

The idiom often conveys a sense of frustration, as you feel like you're so close to remembering something, but just can't quite get it out.