What does 'sync' exactly mean when using iTunes?
Synchronization is the process of ensuring that your iTunes and iPad/iPhone contain the same up-to-date files. If you add, change, or delete a file from one location, the synchronization process will add, change, or delete the same file at the other location.

For example, if you had a few songs on your computer, and a few on your phone, and the ones on your phone were not on the computer, and the ones on your computer were not on the phone, then after a "sync" operation, the contents of your computer AND the phone is the combination of contents of both devices. 

In short,  the content on device 1 now exists on device 2 alongside device 2's original content, and the content of device 2 now exists on device 1 alongside device 1's original content.