What does it mean to feel used by someone?

Sometimes a person will get involved with another person, not for the sake of friendship or to develop a relationship, but to get something. It's true that all relationships involve give-and-take, but the person I'm talking about is good at getting, and not so good at giving. If you are involved with a person like that, who always seems to be getting some kind of benefit from you without coming half-way, then it may be that you are being "used". Don't be too harsh; you need to understand that you probably allowed the "using" to happen.