Songwriters may make money from public performance fees (such as when someone does the song at a concert) and mechanical licenses (such as when a record company pays the publisher for using the song on a CD sold to the public). Another source of income for songwriters can come from synchronization licenses.
The formal name used is “synchronization license fees” but I will use the shorthand term of “sync fees.” When a third party wants to synchronized visual images with a musical composition, the copyright owner may agree to permit the use in exchange for sync fees.
The syncing does not need to involve precise synchronization of the music to video images, such as when lip-syncing. Syncing in this context merely means that the music coincides with the visual images in some way.
Examples of syncing include music videos, songs in movie soundtracks, and instrumental background scores for movies and television.
The music publishers often split sync fees with songwriters on a 50% basis.
The copyright owner has the right to refusal, unlike compulsory mechanical licenses. Thus sync fees depend on how much the copyright owner wants to get and how much the user wants to pay.