Relational database
A database that stores data in tables with rows and columns, and links the tables to each other.
What it tells you about a developer
Relational database work is the most transferable data skill there is. The model has been standard since the 1980s, so experience with one relational database carries to the others with a short ramp. A developer who can describe how they designed tables and the links between them is showing the judgment that keeps a product's data trustworthy as it grows.
Relational databases organize data into tables: a customers table, an orders table, and links between them. Rules built into the database keep the data consistent, so an order can never point to a customer who does not exist. Postgres, MySQL, and SQL Server are the major ones, and they hold most of the world's application data.