Schema

It's the shape of your database on paper, including what sort of value each column takes. In your project it's one file.

You'll meet it the first time the app has to remember something new, because a field doesn't exist until the schema says it does. That's why "just add a box to the screen" turns into a database change.

The rule to hold on to: you edit the schema file, and a migration carries that change to every copy of the database. You never reach in and change one by hand.