What does it mean when a girl says don't worry about it when you tell her you like her?

This is a somewhat indirect kind of comment. You didn't say that you were worried, so why should she say don't worry about it? I would interpret this to mean, it doesn't matter very much whether you like her or not, she is willing to associate with you to some degree but not to a very close degree, and she won't be an important part of your life.