Synchronization in a recording contract is when the recording artist's music is synchronized to a video: music video, movie, television, commercial, etc. The record company can receive all the money from that if the contract states the company gets all synchronization rights. As an artist, he/she would not want the company to have full synchronization rights and would want to get paid royalties for that just the same as album sales.
A synchronization licence is needed for a song to be reproduced onto a television programme, film, video, commercial, radio, or even an 800 number phone message. It is called this because you are "synchronizing" the composition, as it is performed on the audio recording, to a film, TV commercial, or spoken voice-over. If a specific recorded version of a composition is used, you must also get permission from the record company in the form of a "master use" license. The synchronization royalty is paid to songwriters and publishers for use of a song used as background music for a movie, TV show, or commercial.