|The 180 Degree Rule. Wikimedia|
"Crossing the Line" is also known as the 180-degree rule, and it is one of the fundamental rules of cinematography.
Like most rules of film-making, the only way to really understand the 180 Degree Rule is to break it, figure out what went wrong - and then work out how to fix it next time.
Let's say you have a shot with two characters talking to one another. Draw an imaginary line between the two characters. This is the "line" that we should not cross.