It means to hope that something will take place, that your desire or wish will be fulfilled. example How did you do in the exam? I think I passed, well fingers crossed. you can find more explaination in this site. http://forum.wordreference.com/showthread.php?t=1104847

You say "fingers crossed" when you hope or wish for something. For example: "I think I'm pregnant! Fingers crossed." If you want to, you can actually cross your index fingers and middle fingers when you say it. According to old Christian beliefs, it brings good luck and wards off evil spirits or witches.

Fingers crossed accompanied by the same action means that that person hopes that things will turn out the way he/she wants them to. Example: Q.Do you think that you'll win the lottery? A. Fingers crossed!

It's been a while, but I think when I was a kid it also meant that someone wasn't supposed to be lying (you know if their index and middle fingers were crossed). I'm not exactly sure how this is supposed to deter any would be liar but that's kind of how I remember it. And also, it's for good luck, supposedly.