Crossing your fingers is a hand gesture commonly used to superstitiously wish for good luck. The gesture is referred to by the common expression "keeping one's fingers crossed" or just "fingers crossed," meaning "let’s hope for a positive result".
The gesture has also been historically used in order to allow believers to recognize one another during times of persecution. Some people, mostly children, also use the gesture to excuse their telling of a lie. It may have it's roots in the belief that the power of the Christian cross might save one from being sent to hell for telling a lie.
A similar belief is that crossing one's fingers invalidates a promise being made. The gesture is also used to express two people being close friends with the accompanying phrase, "They are like this." In 16th century England, people crossed their fingers to ward off evil. They also did it when someone sneezed or coughed.