The 180º rule is a general guideline regarding the on-screen spatial relationship between two characters within a scene. The rule involves keeping the camera on one side of an imaginary axis between the two characters.
In effect, the first character is always frame right of the second character. If you were to cross the imaginary axis with the camera, this is known as “breaking the 180º rule” and it visually confuses the audience as to where the characters are standing in relation to each other.
Otis Morley’s A-Level Film Studies Blog 