definition and description
A close up shot usually shows the face in detail so that facial expression can be seen.
The shoulders are usually in the shot, but not always.
If more than the shoulders are shown, then it would be a mid-shot.
director's purpose
The director tightly frames the face to share the character's emotion with the audience.
impact on audience
The audience responds to a close-up shot by empathizing with the emotion of the character and feeling the same as them. In this way they engage with the film.