MEMC allows TVs to play video more smoothly.
MEMC helps reduce jitter or motion blur in video by inserting frames between existing frames. It can make 24/30fps content look like 60fps content.
While it may look attractive when watching sports/series/documentaries, turning it on during movies can have a soap opera effect (you feel like watching a series instead of a movie). I personally turn it off and turn it on only when needed.