The term ”medium format" refers to film formats between 35mm and ”large format”, as in 4x5” or 8x10” sheet film. Generally this term refers to 120, 220, or 620 roll film, although there were many other formats produced over the years that had similar sizes.
120 film is the most popular and long-lasting medium format.. format. Cameras designed for 120 film take images in a variety of formats, from 6x4.5cm to 6x9cm or even wider in some niche cases. The image quality of these cameras vary quite a lot, but the larger negative size means more detail is being captured than with 35mm film or other small formats.
For a long time medium format was the standard for all photography, and remained the go-to for professionals despite the success of professional 35mm cameras like the Leica M3 or Nikon F.
While medium format does generally offer a significant image quality boost over 35mm, it comes at the cost of size, weight, speed, and price. Medium format cameras and film will generally be heavier, bigger, slower, and more expensive than 35mm cameras.
For some, this tradeoff is worth it. If image quality is your number one concern, a medium format camera may be the best choice for you.