A split edit is a transition from one shot to another in film or video, where transition of the audio and video happen at different times. This is often done to enhance the aesthetics or flow of the film, allowing the audience to see context—either before or after—of speaking rather than simply the speaking itself. Without split edit, a conversation between two people can feel like a tennis match. Different variants of split edits received their own names based on how the video being edited is presented on the timeline including J Cut and L Cut.
Split edits are also used to hide transitions between scenes. They can be very effective in editing dialog scenes shot with a single camera using multiple takes. The ability to cut the picture/video separately from the sound/audio allows the sound from the various takes to flow smoothly, even though the picture cuts are at different places. Read more at Wikipedia.