You should better not manually delete any of those files… that’s where Plex keeps its database as well as any media assets stored for your videos (e.g. posters, video thumbs…).
https://support.plex.tv/articles/202529153-why-is-my-plex-media-server-directory-so-large/
If you want to free up some space you should let Plex handle its own files.
The biggest impact is probably to disable Generate chapter thumbnails (see Settings > [Server Name] > Library) or to move some temporary folders off that drive (e.g. by setting Transcoder temporary directory in Settings > [Server Name] > Transcoder).
It’s possible to move the entire data folder off your system drive – however keep in mind that every file that’s not accessible because Plex first needs to spin up an external or network drive will slow down your Plex Media Server!
