What does it mean that a camera is a 35mm?
That is the film size used in the camera, I.e. a 4x5 camera uses film that is 4" x 5" an 8 x 10 camera uses film that is 8 x 10 inches. Medium format cameras use 120 film which is 70 mm. and it provides many different options. a roll of 120 film can yield 8 pictures per roll or 10 or 16 or 12 depending on the format of that camera. So a 35 mm camera uses film that is 35mm wide!
I hope that is sufficient information to your question.