EYELINE MATCHING is a technique where two shots are edited together to make it appear that one character is looking at another character, even though two takes may have been recorded at different times. Matching eyelines create continuity of space and time for the audience.
Imagine a conversation with two characters. Both characters are framed in single shots, looking off-screen. Even though the other character is not in the single shot, the direction of their eyes suggests that the characters are looking at each other when edited together.
During production, it is important for the crew to ensure that eyelines match from one shot to another in a scene. Otherwise, the two shots may violate the 180-degree rule and/or cause confusion for the audience.