The 180 Degree Rule

In film making, the 180-degree rule is a basic guideline regarding the on-screen spatial relationship between a character and another character within a scene. 

An imaginary line called the axis connects the characters and by keeping the camera on one side of this axis for every shot in the scene, the first character will always be frame right of the second character, who is then always frame left of the first. 

If the camera passes over the axis, it is called jumping the line or crossing the line.

This helps to make sure the audience does not get confused when two characters are in conversation.