Synchronization Licensing is literally 'Contracting' to 'License' authorization to 'use' the Intellectual Property (IP) of the 'owner' of a Song or Composition.
It could be for the composition as written in manuscript form, the entity seeking the Contract intent on hiring musicians to play it, the Sound Recording (SR) of it, or the 'owner's own Sound Recording, to 'synchronize' with video 'content'.
A massive market, video content, on a global scale, Synchronization is the background music in a Tv or Radio commercial advertisement, or soundtrack for a Tv show or movie, documentary. Any 'use'.
The 'use' can be Contractually specified as to how long the Contracting entity desires some degree of exclusivity, before the Contract ends and the owner is free to market the work to other 'users'.
Depending on your given situation, you can, as John explains, have your Publisher handle those details, with you approving the terms and affixing your Signature to the Contract, or your Publisher authorized to make the 'deal' without your review.
If you are self-Publishing you have to negotiate the Contract with the 'user', making sure it says what the terms are. They may know more about it than you do and 'negotiate' a 'deal' favorable to themselves, perhaps more 'favorable' than you want to give.
Again, consultation of any Contract with any Lawyer is better than no Lawyer. You want a Contract that has a Reversion Clause, to make sure it ends at some point and the right to market the product, the Song, or composition, the Sound Recording, the Master, and the Copyright are unencumbered. It remains your Intellectual Property to do with as you please.
The dollar amount pays for the 'use', for the specified period. No further 'rights' are offered or agreed to beyond what is specified in writing.
Be congenial. Keep the negotiation moving forward. They want your IP. You want them to have it in exchange for the fee. You want to retain ownership of the Master Recording and the Copyright, rights to Songwriter Royalties, Publishing Royalties, unless the Publisher is seeking 100% or some percentage of the Publishing Royalties to be collected. Keep it moving toward getting paid, without giving up more than that payment is worth.

There will always be another song to be written. Someone will write it. Why not you? www.garyeandrews.com