A medium shot is tighter than a medium wide shot and wider than a medium close-up, though they sometimes cannot be exactly distinguished.
It usually contains one or more persons from the waist up, including their closer environment.
How medium shots can be used
If a character should be contextualized with their immediate environment, a medium shot is a suitable size.
Similar to medium wide shots, medium shots are fit for group shots or two shots.
A person in their immediate surroundings
If you want to establish a character interacting with their immediate surroundings and still recognize the facial expressions, a medium shot would be the way to go.
Even if the person in your frame is not interacting with something close to them, the medium shot offers the possibility of contextualizing character and setting.
Two shot or group shot
When people are shown from approximately the waist up, the expression of the upper body adds to the visual content of the shot, as well as the emotions indicated in the facial expressions.
The medium shot is wide enough to enable you to visually deliver the dynamics of the relationship between the characters through their positioning in the frame, along with their body language.
Transition between wide and close shot sizes
Medium shots are often used in editing as a transition between a wider, more expository shot and a closer, more intimate shot or vice versa. Thereby, you can influence the audience’s emotional involvement with your characters.
Of course, it is also possible to edit directly between a long shot and a close-up, but a medium shot in between might increase your narrative opportunities.