Lip syncing is where the artist themselves or an actor will move the lips silently in synchronization with a recorded soundtrack. Artists don't just do this in their music videos, they can also do it in live performances.
Bad lip syncing in music videos can be very distracting for the audience. It will make the video look silly and poor quality. Where we are so used to films having lip syncing spot on, when we see it not fitting it gets picked up straight away. Even in live performances, If you can't lip sync perfectly it can actually upset your audience as they expect their idols to sing live not mime.
Choose a song you love. If you're in love with the material you're working with, that passion will shine through on stage. Choose a song that you're passionate about and that has personal meaning for you when lip syncing.
Use a mirror. Practice singing along to the song in front of a mirror. Watch how your lips move. See if they're in sync with the song. You may find that you need to slightly exaggerate the movements of your lips and mouth to make it look like you're singing.
Pitch Perfect is a perfect example of good lip syncing. It is actually hard to believe that it is all lip synced! They recorded the actors singing the songs however within the actual film it is just them lip singing!