> What does it mean to turn any set of notes into a powerful database?

You must be fun at parties. Complaining about everything, but not even bother to read the damn manual... It's explained on the third sentence on that site. Ok, to be fair, there is a big picture between those parts, and you have to follow some links for more details.

> And why set of notes?

Just curious, do you even know Obsidian? Have you ever used it? You read like someone who has no clue about this software, jump right in the middle of the manual, and then complain that you missed the tutorial.

Obsidian is a markdown-editor with knowledge base. Notes are its lifeline, and they have since nearly the beginning the option to put metadata into each note in a special section (in yaml), basically the header of the note. This metadata are now called properties. Bases is a feature building up on this metadata, offering a database-like experience for viewing and editing them in a specialized UI. The database, is the vault, the folder+subfolder containing the notes. Each note is a row in that database.

This is all explained in the documentation, if you just would read it...