The 180 Degree Rule is an important aspect of the film grammar. Not following the 180 degree axis will be very confusing due to the fact that the characters will face each other from the opposite side. An simple example of this would be two different characters facing each other and communicating and then the camera angel changed to the other side making the viewer wonder if the characters had changed seat's.
if the filmers crosses the 180 degree axis, it is called crossing the line. And the new shot from the opposite angel is called reverse angle.
In a football match if the red team is attacking from left to right it makes perfect sense however if the camera cuts and starts filming from the opposite side it will look confusing since it would seem as the team is attacking from right to right.
