In American schools, a staff room is usually a small workspace that is officially off-limits to students, although enforcement of this depends on the school. Most staff rooms will have a sink, coffee maker, refrigerator, microwave, copier, printer, paper cutter, stapler, three people trying to share the one table for work, and one person messing about making food/coffee/a mess and trying to stay out of the way. The exact details depend on the school, and some staff rooms will have fancy things like laminators or soda and snack machines.

I work in poor schools in a poor state, so our staff rooms also have educational testing, reading coaches, occupational therapists, physical therapists, ESL teachers, and etc., in them -- along with their assorted book sets, easels, balance beams, balls, and spare wheelchair parts. There are also universally a few cabinets that are either locked or full of something surprising. Because these rooms are important and crowded, most schools built in the last few decades have multiple staff rooms, usually one for the main office and one for each wing. Even so, it is often hard to find one that isn't full of people doing something important, resulting in many teachers being locked away from their lunches/coffee/printer/copier.

When in doubt, the office staff room is pretty much always the nicest and the least likely to be occupied with someone doing student testing (because the principal keeps their preferred caffeine source in there), so stick your lunch in that fridge. It will also have the fanciest coffee maker, be the most likely to have a snack machine, and be the cleanest. On the other hand, the office staff room will probably not have any office supplies, as the office will probably have a dedicated copy room.