What does it mean when a straight girl wants to be a guy?

Sometimes, a person feels as if they are born in the wrong body. This has happened to boys as well girls. But since you asked about a girl, let me answer it to only apply to a girl. While there are girls who are what society calls "tomboys" (they identify with and enjoy hobbies and sports that are gendered masculine), this does not lead to any major physical or psychological changes-- the girl continues to identify as a female but simply enjoys those things that we associate with males in our culture. You are perhaps asking about the small number of girls who seriously believe nature made a mistake and they were supposed to be male. After counseling and consultation with doctors, some of these girls do in fact decide to undergo "gender reassignment," a process that will lead to becoming a male-- it requires surgeries, and taking the hormone testosterone. Such a person is called "transgender." I enclose a link from the American Psychological Association.