When two people talk face to face, there's an invisible 'line' between them. As long as the camera always stays on the same side of that line, when you cut the shots together A always looks right and B always looks left, and the audience instantly understands who's talking to whom.

But the moment you move the camera to the other side of the line (called 'crossing the line'), the two suddenly look the same direction when cut together — as if talking to themselves, or with their directions scrambled. This is the culprit behind a rookie's dialogue scene always feeling off.