An individual Tinderbox document is simply a single computer file in which Tinderbox stores its data (for the more tech users, it's data stored in XML form). A Tinderbox document (file) can contain one note - hundreds or thousands of notes - see more.
Most users, especially when starting out, will have only one Tinderbox document. However, depending on needs, some users may have several individual documents, perhaps for different discrete topics. Note: there is no 'correct' number of documents to have - it simply depends on a users' needs or personal style of work.
These Tinderbox documents are often referred to in support documentation and the user-to-user forum as 'TBX documents' or 'TBX files' or just as 'TBXs' in passing reference to the fact the files are stored with a '.tbx' file extension.
It is worth noting that Tinderbox can open more than one (TBX) document at the same time. For such users, Tinderbox tracks recently opened documents on a sub-menu of the File menu. If multiple documents are open, the application's Window menu list all open windows from all documents. Note that the latter listing doesn't indicate the document to which each document belongs
For users who only ever use a single file there is a useful (app-level) preference to "Reopen last document at startup". As only a single Tinderbox document is ever opened, it will automatically be opened whenever Tinderbox is launched.