definition
An extreme close up shot shows the detail of one particular thing. It will only show part of a face rather than a full face.
director's intention
The director uses an extreme close up shot to first of all help tell the story and secondly to create interesting images for the viewer as they watch the film.
impact on audience
This type of shot causes the audience to think about what might happen next or wonder what the character/creature is looking at. The audience might think in this shot for example that the snake is looking at something which it wants to attack.