The 180 Degree Rule is an important one to follow when shooting two or more people in a scene. Essentially, it helps the audience watching the film or video keep the characters straight in their minds and oriented in physical space.
This rule draws an imaginary line through the scene; one that the camera should not cross. By keeping the camera on one side of the line (essentially up to a 180 degree arc), eye lines remain constant. This lets the viewer feel more comfortable and makes the relationship between subjects clearer. The man on the left will always stay on the same side of the screen if the camera maintains its position on one side of the arc.
When you start a scene, try to draw a virtual line through the scene. Its up to you to decide where to place the line originally, but you shouldn’t change it once you start rolling. If the camera physically crosses the line for a consecutive shot, the direction in which your subjects are looking or moving will seem to flip in the camera. This can be very disorienting to the audience and should generally be avoided.