The 180° rule is a basic film/video editing guideline that states that two characters (or other elements) in the same scene should always have the same left/right relationship to each other. If the camera passes over the imaginary axis connecting the two subjects, it is called crossing the line. The new shot, from the opposite side, is known as a reverse angle.
Here is an excellent video example of the 180 degree rule from Videomaker magazine. Jennifer O'Rourke is the managing editor and has produced many great tip videos for her program.
Some filmmakers state that the fictional axis created by this rule can be used to plan the emotional strength of a scene. The closer you place the camera to the axis, the more emotionally involved the audience will be.
This video from Sonnyboo explains graphically the 180 degree rule and it's effect on the viewer.
But remember, all rules are made to be broken. If you want to throw the audience off then crossing the line could be a dramatic way to disorient the viewer. But be sure that this is the effect you want to achieve. In most cases, you will never break this rule. Enjoy the videos and watch your position in the viewfinder. ;-)