A workspace is just a folder on your disk. When you launch Nota for the first time, it asks you for a folder to use. This folder them becomes your current workspace. Most people would use a folder that lives in their Dropbox or OneDrive, and most would call it Nota, but you can use any folder on your hard drive, and it doesn't have to be empty — it can already contain .md or other files.

You can change your workspace folder from File → Open..., and this folder will become your new workspace. You can then switch between workspaces from File → Switch Workspace... ⌃r.

When you first launch Nota, it asks you to set a workspace folder (or workspace for short). This is a regular folder on your Mac where Nota stores its documents. You can select any folder on your hard drive, and it can be a folder that already contains (.md) documents.