1. Literally, to cook hamburgers, typically but not always as part of one's job at a fast food restaurant. (Cooking hamburgers often includes flipping them on a grill or flat top to ensure each side is cooked.) I spend all day flipping burgers, so I'm never in the mood to eat them when I'm not at work. Joe's out back flipping some burgers on the grill. If you're hungry, I can tell him to throw another one on.

2. By extension, to work at a job that is low-paying, monotonous, has few opportunities for advancement, and is below one's potential. If the phrase refers to a hypothetical job, it does not have to be limited to fast food. However, this phrase is not usually used to refer to an actual job if that job does not involve fast food or literally cooking hamburgers. I know you need cash, but I think you should focus your energy on finding a solid job rather than picking up a few bucks flipping burgers. A lot of people look down on flipping burgers, but that's where I started, and I've worked my way up to regional manager.