On windows, it’s %APPDATA%\Obsidian\ObsidianCache. There is one json file per vault.
We store filenames, headings, tags, linked mentions positions, etc. All these things are used to support\speed up the various functions provided by Obsidian.
You don’t need to backup/move the cache file. If one is not present, or it is outdatad, Obsidian will crawl your vault and generate a new one.
Moreover, some configuration settings are stored within each vault in a .Obsidian subdir.