The hint is in the folder name.
You or one of your shared users with ‘Sync’ privilege has initiated a sync of (probably) several items (or even maybe a whole library) to his/her local Plex client.
(Plex for Android, Plex for iOS and Plex for Windows/Mac can do this)
If it was you who acidentally did this, you can find (and delete) pending Sync jobs in the web app https://app.plex.tv under Settings - Server - Status - Sync
If it wasn’t you, who triggered this, your best way to find out who did it is to install Tautulli