A space is SilverBullet terminology for a workspace, or project, or instance. Obsidian calls this a vault, LogSeq calls it a graph. You may think of it as a Folder or a directory — because in practical terms, that’s all it is.
Feel free to back up or manipulate your space’s folder and its files with whatever tool you like — you don’t have to use SilverBullet exclusively. You may want to turn your space’s folder into a git repository, for instance, and do version control and back-ups that way.
A space consists of
A space belongs to whoever runs the server it lives on. Often that is a single person — but nothing about a space assumes it. A space may just as well be held by a handful of people, each with their own account and access level, together with scripts writing to the same files.
Every space in SilverBullet at the very least has an index page (by default named index.md). Once you configure something, it also has a configuration page, ^Library/Config (the file Library/Config.md), so most spaces have a Library/ folder. Libraries you install go there too. Spaces from before this change had a root CONFIG.md; the server moves it to Library/Config.md the first time it starts.