Database
Software built to store an application's data reliably and find it fast: accounts, orders, messages, everything an app remembers.
What it tells you about a developer
Database depth is a durable, highly transferable backend skill, because schemas and query performance outlive framework fashion. Postgres experience in particular travels almost everywhere. A developer who can talk concretely about a slow query they fixed or a schema they redesigned is showing senior backend judgment.
Nearly every application keeps its permanent data in a database. The names to recognize: Postgres and MySQL are relational SQL databases and the default choice for most products. MongoDB is a document database. Redis keeps data in memory for speed. Database work sits on the backend, and how a developer models data reveals their engineering judgment.