Database
A database is an organised place where an app stores its information, like customers, orders and bookings.
In a simple example
When a customer books a table, the app writes the booking into the database, and reads it back when you open the list.
Why it matters to you
Without a database an app forgets everything when it closes. With one, it remembers.
Do not confuse with
A database is not a spreadsheet file you open. Programs talk to it, and it can hold millions of rows safely.