The cafeteria in the physics department at Caltech is where the guys eat lunch, and occasionally breakfast (0206) and even late-night snacks (0205). A dietician who works there is Olivia Geiger (0103). One of the lunch room staff whose "English is not that good" sat in on Sheldon's Physics Bowl team (0113).
The cafeteria was first shown in Jerusalem Duality, the 12th episode of the first season.
Occasionally actual scientists sit in the cafeteria set on episodes of #bigbangtheory. No reason for it -- it's just fun to do ... they're fans who work at local universities. We've had some from places like Harvard as well.