Database.

A database is where an application stores its information so it can be retrieved, updated, and queried reliably. Every account, message, transaction, and setting in your product lives in one. It's the memory of the application.

Choosing and designing a database well matters more than it sounds. A good data model makes features easy to build and the app fast; a poor one makes everything harder and can be painful to fix once there's real data to migrate.

Databases come in families: relational ones like PostgreSQL that store structured tables, and others suited to different shapes of data. The right choice depends on what you're storing and how you'll use it.