What does it mean for a girl to send me an expression pack of "Bring it on"?

Many times it doesn't really mean anything. It's just an expression pack. Maybe sometimes. The speaker has no intention, but the listener has a heart. So, don't think too much. Maybe it's just cute or like this expression. Generally speaking, this expression means yes, coming. Yes, maybe he doesn't mean anything in particular, but his expression is funny. I suggest you don't think too much.