The 180 degree rule is the basic guide line concerning the on screen spatial relationship between a certain character and another character or object that is in the 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 right of the frame to the second character, who is then always left. If the camera passes over the axis, it is called crossing the line or jumping the line. A re-establish shot is used if the character move positions.
This video helped to understand more of how to use the 180 degree rule within film making.
