Lip-syncing is when an actor or singer moves their lips silently in synchronization with a pre-recorded soundtrack. It is used more often than not in the production of music videos but sometimes it is also used, "controversially", in live performances.
It is used to give a more polished effect in the music video as it makes the audio sound smother as it has been pre-recorded. It also is used to reduce the risk of background noise in the soundtrack as all audio that is picked up on set isn't used in the video itself.
It is also used for visual improvements. This is because if an artist is not singing then there is no need for them to wear a microphone pack. Furthermore it is also used so that a dance routine can be performed without the artist having to focus so hard on their breathing, as a result this creates a better spectacle.
