Synchronization license is just that, a license you issue someone (producers, record label) enabling them to "synch" your audio to a video, music video, television, commercial etc. In most cases a record company owns the master to the song so they receive all monies created from the license and the artist is paid royalties based on whatever agreement he/she has with the record company beforehand.
If you used your audio in a video, commercial or even on a 800 number line, you must have licensed that to the producers of the visuals that are being synched to your audio. In some cases a certain audio recording of a composition is used and requires you to obtain a "master-use" license from the masters owner. Royalties are usually the same as album sales royalties but all that should be outlined clearly in your agreement/contract.
It is wise as a videographer, photographer and even the editors of the content to be sure they have written permission or an official synch license before working with the audio to ensure not to be involved in a legal dispute. A license does not have to cost you anything, all parties just need to clearly state all aspects of the collaboration and have it signed and notarized. Basically if those steps are not taken, the owner of the visuals will in turn own the video and all content included within it(audio included).
Licensing your music is a great way to get exposure and turn a dormant catalog into a very profitable one. Instrumentals can be bought by various companies and businesses and many others in need of music and audio clips. So be sure to take the proper steps to secure your content through licensing.
