There is an old custom that says, if while making a wish you "cross" (intertwine) your index finger and middle finger, the wish will come true. So when we say "keep your fingers crossed" it means that you should hope that your wish comes true or that very good things happen.

This is a figurative expression of a force to attract good luck or good wishes

hope for best

You put your finger on top of the other one and this is the sign of "hope for good luck".

It is an expression, or a behaviour used for prevent misfortunate event.