This bite-sized video explains the 180-degree rule in eye-line scene direction. If you’re just getting started as a director, this rule is important to follow so that your audience isn’t confused by visual cues you don’t intend - such as switching back and forth between two characters in dialogue that seem to be standing on the same side but in fact are face-to-face.
As the good folks at Film Riot explain - you can break this rule, intelligently, but first, you need to understand it.