Files, directories, users, groups, other users, ownership, permissions. Streuth!
This is fundamental Tux, or Linux, stuff and, regardless of your self-hosting type, you have to live and breathe it because, if you're not clued up about your server's permissions structure, and in turn the security of your web files, you're begging for trouble. Here's the deal.
Linux is a bunch of files. Everything is a file. Even directories are files, listing other files.
Actually, two things are not files, users and groups. We'll consider those now.
When a user is created, a namesake group is also created by default. So you may have the username superbob and be a member of the group superbob.
When user superbob creates ...