Today, that girl told me that she was interested in me and that I didn't like her. She was joking. What does she mean by saying that?

What a girl means is that although she likes you, she thinks you don't like her, and then if she doesn't like her, she can ignore it. In other words, she doesn't like you, and she likes you to the point where she has to marry, so I think this girl has a good impression on you at present. It should be like this. If you are interested in her, you can talk. Better not be together, that's all.