The 180 degree rule is a concept used in cinematography to keep the audience oriented to the space of a scene. The basic idea is to draw an imaginary line in down the center of two subjects that are talking to each other and keep the camera on one side of that line. This helps avoid confusion on where the characters are in the scene and which way they are facing.
This video highlights the 180 degree rule:
This rule is more like a guideline for newer filmmakers, but can be broken: