What I learnt about the 180 degree rule, in class, through research and fom watching examples on websites such as youtbube, is that it is an imaginary line that cuts through a scene. When filming a scene, you must not cross this 180 degree line as it would cause disorientation for the audience watching it. Therefore, you must keep all the cameras (if using more than one) on the same side. Here is a diagram I drew to help understand the idea of the 180 degree rule: