I know what you mean...
You say you get it, but you're actually just trying to cover up the fact that you didn’t get a single word they said.
They explain a math problem. You say, 'I know what you mean,' even though you have no idea what the problem is about.
They tell you a complicated story. You say, 'I know what you mean,' while secretly thinking, 'What even happened?'
They talk about their weird new hobby. You say, 'I know what you mean,' when you're just trying to pretend you're into it too.