"Keeping your fingers crossed" means that you are hoping that something good happens in a situation. Put your middle finger over index (pointing) finger. This is crossing your fingers.
I believe it is linked to Christianity- the sign of the cross is for protection. People cross their fingers for luck. If you bet money on a horse race, you might cross your fingers during the race, hoping your horse would win.
See this link for various images of crossing fingers:
"crossed fingers" - Google Image Search