Saying that someone is "one of those" means that the person is bad, annoying, difficult to deal with, etc. You use it like this:

A: Whenever my sister travels somewhere, she packs three huge suitcases and her poor boyfriend has to wheel them around for her.

B: Ah, so she's one of those.

So you can think of this as meaning "She's one of those (difficult or annoying people)."

This might seem like a rude thing to say to someone, but people usually use the phrase "one of those" in a friendly, teasing way. You can even say this directly to someone you've just met:

You're one of those, aren't you? I can tell!