What do you think this means? Let's listen.
Keep my fingers crossed.
A: Hey, How are you?
B: Hey, I am doing great. Actually, I just had a great job interview the other day. I really hope they hire me.
A: Oh, have you heard back from them?
B: No, not yet. But I'm keeping my fingers crossed. Hopefully they'll call me today.
They sound very hopeful, and that's exactly what it means.
In American English, keeping your fingers crossed means that you are hoping for the best possible outcome. This is a happy expression, and you can use it in any situation -- formal an informal -- and everybody will understand what you mean.
And that's English in a Minute.