Database
aka: dbaka: sqlaka: postgres
The Quick Version
The thing that remembers information after someone closes the tab.
How it actually works
A database is structured, permanent storage. Data lives in tables with rows and columns, and your backend asks it questions ('give me every order from this customer') using a query language such as SQL.
Why does it matter?
Without a database your website forgets everything the moment the page reloads.
Example
User accounts, blog posts, orders and bookings all live in a database.