180 Degree Rule
The 180 degree rule refers to keeping a camera positioned on one side of an imaginary line, this is to achieve the best shots that make sense to your audience, helping them gain an understanding of where everything is in the scene. This can be achieved through using looking space and eyeline match.
In addition to this, it is important to capture facial expressions and reactions of the actors in order for the audience to gain a sense of what's going on in the scene, this is where you shoot in one direction (facing over the shoulder of one character) and then do the same with the character facing opposite.
Here is an example of a shot reverse shot, it is first shot showing one direction and then the exact same angle in the other direction. This is to clarify to the audience that they are exactly looking at eachother. In order to correctly achieve this, you need to imagine that there is a line between the actors and you must not cross that line by moving the camera too far away.
Here is an example of the line (the blue area) that must not be crossed, if the camera is placed outside of the line, then the rule is broken.