When someone says they keep their fingers crossed, it means they wish or hope for a good outcome for someone or something.
Where does this idiom originate from? Why do people fold their one finger on another to hope for the best?
From the New Oxford American Dictionary:
cross one's fingers (or keep one's fingers crossed): put one finger across another as a sign of hoping for good luck. • hope that someone or something will be successful.
The origin of this hand gesture is discussed, for example, in Wikipedia. It is not fully clear, but probably comes from pre-Christian times, and thus does not have a relation to the Christian symbol of the cross.