Not exactly, considering prayer is religious. When someone says keep your fingers crossed, it is superstitious in nature, but not religious. The phrase is usually uttered (at least in my experience) when a person is making a gamble on a result of something.
Let's say I am trying to repair my computer, and I am not exactly sure if I have correctly repaired it, and someone is with me. I may say "keep your fingers crossed" and the other person might literally cross their fingers.
It's almost like the phrase "Knock on wood", except "Knock on wood" is what a person does if they do not wish to ruin any progress that has been made if you remark on the progress.
"Wow, my computer is running faster so far"
"Knock on wood"
After which chances are, someone will actually knock twice on a wood desk.

I guess it's everywhere in the world. In french it's "croiser les doigts" (je croise les doigts). I would add also "and also the foot-fingers!")

"Keep your fingers crossed" very simply it means: to hope that things will happen in the way that you want them to.

The expression   you refer to is this;   "Keep your fingers  crossed."

 It is based upon  the belief that if you keep 2 fingers crossed it expresses,  a hope, a  wish,  or brings good luck to you when you want a specific result

Thanks a lot !Is this a costom in America?