An imaginary line called the axis connects the characters, and by keeping the camera on one side of this axis for every shot in the scene, the first character is always frame right of the second character, who is then always frame left of the first.
The camera passing over the axis is called jumping the line or crossing the line; breaking the 180 degree rule by shooting on all sides is known as shooting in the round.
The 180 degree rule enables the audience to visually connect with unseen movement happening around and behind the immediate subject and is important in the narration of battle scenes. Read more at Wikipedia.
