180 Degree Rule
The 180-degree rule states that two characters (or more) in a scene should always have the same left/right relationship with each other. This rule is in place to help the audience understand the geography of the space.
The rule dictates that you draw an imaginary line between these two characters and keep your camera on the same side of this 180-degree line. The idea is as if you were a person in the room, these are the views you would see by turning your head left and right.
If your camera crosses this line, your audience’s understanding of where the characters are and their left/right orientation will be thrown off. And unless you’re intentionally trying to exploit that, it makes things look confusing, messy, and unprofessional. You are able to cross the line if you show the viewer that the camera is crossing over the line, this can be done with a dolly move or handheld move. Once you have crossed the line, your other cameras must match the new line you have created. This is often done to create slick feeling sequences with lots of movement.
This is a great breakdown of how screen direction and line crosses can create confusion in a sequence.
https://www.openculture.com/2011/12/anatomy_of_a_flawed_action_scene.html
Breaking the 180 Degree Rule
If you seek to create confusion or disorientation, breaking the 180 degree rule could help you to bring about that effect. A great example is in The Founder, when Ray Kroc first visits a McDonalds. The experience is so confusing to him as to how it all works and the filmmakers help show his mind working through it by intentionally crossing the line before he walks away from the counter.