Postgres, SQLite, etc are tools for database management, things like user data, application data and so on are collected here.

Edit: the DB_PASS=“postgres” is the default password when setting up a Postgres database.

Take Reddit’s Karma system or Upvotes/Downvotes for example, they’re stored in a database and however Reddit wants to utilize that data Postgres makes it easy to call upon it.

I’m sure others can give more detailed responses, I’m typing this out in a rush.