Yes, keeping one's fingers crossed in our Western culture is a protection against bad luck. I speculate that is is related to the same thinking by which we ward off evil (e.g. vampires) with crosses. The cross in Western culture remains a symbol of hope, grace, and good luck.

it should be fingers. There's a nice picture here: http://lisamariaartista.com/keeping-my-fingers-crossed/

It means hope for good luck