This is a supertitious phrase that we use when we are waiting and hoping for some good news. For example, when you've just had a job interview, and this is the job that you really want, you can say, "I think the interview went well. I really hope I get the job. I'm keeping my fingers crossed." While you're saying it, you cross your middle finger over your index finger. This is supposed to bring good luck, and a long time ago, was believed to keep away evil spirits.
Crossing your fingers is also used when you're making a promise that you don't mean to fulfill or when you say something that is not true. This is usually done by children, and they usually cross their fingers behind their backs so that the person they are talking to doesn't see it. For example, if a child promises to eat his vegetables when he's at camp, and he's crossing his fingers while making the promise, it means that he doesn't really mean to eat his vegetables. When he returns home, and his parents ask him if he ate his vegetables at camp, and he says yes while he's crossing his fingers, it means that he's secretly not telling the truth; crossing his fingers means nothing bad will happen to him for lying. Remember, this is superstition.
A: So, how was your exam?
B: I don't know. I really doubt I did well.
A: Yeah? But you studied. I'm sure you did better than you think.
B: Well, I feel like I studied the wrong things. Anyway, I'm keeping my fingers crossed.
A: I hope you did well. You deserve it; you've worked hard all semester.
B: Thanks. Oh yeah! How did your interview go?
A: I think it went well. Thanks. I'm keeping my fingers crossed on that one, too. I really want this job.
B: Well, I'm sure you made a good impression.
A: Thanks. Let's hope so.
Alright folks, think of something you really want to happen--you're getting your car fixed and you hope it won't be too expensive or you have to find an apartment by the end of the week or you hope your girlfriend/boyfriend will like the dinner you're making, ... whatever. Say what you hope while crossing your fingers. "I really hope my car won't be too expensive to fix. I'm keeping my fingers crossed." That's how it works.
Tomorrow, we'll do another superstitious expression. Have a good Monday.