Establishing the Line of Action
The 180-Degree Rule is a guideline in filmmaking for where it's best to place the camera when filming a scene, so that the audience doesn't get confused about the geographical relationship between the characters. The rule states that these characters should always have the exact same left and right relationship to each other in every camera setup.
To establish the 180-degree line, draw an imaginary line from one character's nose to the other, that runs straight through the two characters (not between them). This imaginary line is called the Line of Action (also known as The Line, Axis of Action, or the 180-line.)
Once you have a line of action established, you should only place the camera on one side of the line (left or right).
Crossing the Line
When the 180-Degree Rule is broken, it is called Crossing (or Jumping) The Line. This typically happens because the camera has moved across the Line of Action. Shots that have crossed the line can confuse the audience by disorienting them on the location of the characters in a scene. Characters who were on the left, magically appear on the right.
Imagine a soccer player running with the ball from left to right, trying to score a goal. If we cut to a camera on the opposite side of the field, the soccer player will appear to be moving from right to left towards their own goal.
In the gif above, the 180-Degree Rule is established with the male character on the left and the female character on the right. The filmmaker then cuts a wide shot of both characters, but this shot has clearly crossed the line of action, as the male character is now on the right and the female character is on the left.
Example :
For further reading: what-is-the-180-degree-rule