The part of your site that holds everything.
THE SHORT VERSION
Your website has two halves. Files are the code, the design and the uploaded images. The database holds everything else: your pages, posts, settings, users, orders, comments.
A backup of files without the database restores an empty shell.
WHAT LIVES IN IT
Page and post content Settings and configuration User accounts and passwords Orders, customers and stock, in a shop Comments Plugin data
HOW IT IS ORGANISED
Tables, each holding rows of one kind of thing. A posts table, a users table, an options table.
Each row has columns: an id, a title, a date, the content.
HOW YOUR SITE USES IT
Every page load runs queries. The application asks the database for what it needs, receives rows back, and builds the page.
That is why a slow database means a slow site, and why caching helps so much: a cached page skips the database entirely.
WHAT YOU NEED TO KNOW
Where yours is, what it is called, and how to back it up. The rest is useful rather than essential.
