Obsidian Vault
The folder that contains all Obsidian notes. Think of your vault as your Obsidian workspace.
Your vault contains the settings, plugins, & preferences that you have saved, so that your workspace operates the same way each time.
Users can have multiple vaults for various purposes, though I personally use a single vault and organize my folders within it.
You can sync your mobile & desktop vaults if you use the same "root" folder for your vault, but the settings & plugins need to be manually added to mobile.