180 degree rule

 The 180 degree rule is an important concept in video and film production which enables viewers to follow scenes without disorientation or confusion. You can convey the 180 degree rule by drawing an imaginary line between two characters and keeping the camera on one side of them throughout the entire scene. It is sometimes referred to as the rule of line-crossing. Generally, in films the main character is usually on the left side of the screen and the secondary character is on the right, this relationshionship between characters should always stay the same through out the scene. If the camera passes over the imaginary axis connecting the two characters, the rule has been broken because the camera would have crossed the line, thus creating reverse angle which nis not condoned in film making.

Just in case you need a little more understanding or your the type of person who learns more faster through visual interactions, here is a video from youtube by campusmoviefest that explains the 180 degree rule by also featuring an example from Hitchcock's thriller Psycho.