A hand gesture denoting a hope for good luck.
A verbal wish for luck, as in: "I'll keep my fingers crossed for you".
A means of nullifying the binding nature of a promise or oath, as in: "You promised!", "I had my fingers crossed so the promise didn't count!". This is predominantly a childish usage.
A means of allowing or lessening the negative connotations of a lie, as in: "You said your name was Steve, you lied, your name is Paul!", "It doesn't count, I had my fingers crossed!". This is predominantly a childish usage.