With Emby Sync, you can choose specific media, then a device or destination such as a cloud provider along with criteria such as resolution and bit rate and Emby will prepare/convert the media to the format requested, then copy it to the destination.
There are a few different types of sync built into Emby. Some of these sync types are only available to the administrator and other can be used by users with correct permissions.
As an example, an administrator can setup Dropbox or Google Drive as a sync destination, then setup specific media to be synced to the "cloud". In this way the administrator can SYNC popular or new movies/shows to the cloud. When a remote user plays back this media it can be served from the Cloud vs the Emby server saving the server's upload bandwidth. In this scenario the media is prepared and uploaded ONE TIME to the cloud where it can then be played back numerous times.
Another popular use of Sync is by an individual user who has been given access to the sync feature. In this case the user will pick a few movies or TV Shows and sync this content to a mobile device. This allows them to then playback this media "off line".
While these are two completely different uses of Emby Sync (there are others as well) what they have in common is that the original media is not touched in any way.
NOTE: All media always resides on the Emby Server. Syncing does not "move" or "relocate" media in any way and will continue to be available for all other users just as it normally is.
See the Sync Type page for different types of Syncing that can be done and learn how to use this feature.