Many of the backend job adverts we read mentioned databases: SQL, PostgreSQL, MySQL, SQL Server, and sometimes MongoDB or Redis. That makes sense. An app without stored data forgets every customer, every order and every payment the moment it restarts. The database is the memory of the business, and looking after it well is one of the most trusted jobs a backend developer has.
What a database does
A database stores data so that it survives restarts, can be searched quickly, and stays correct even when many people use it at once. Think of a market trader's record book, but one that can answer "how many bags of maize did we sell in March?" in a fraction of a second and never loses a page.
Relational databases first
Most systems start with a relational database such as PostgreSQL, MySQL or SQL Server. Data lives in tables made of rows and columns, a bit like a spreadsheet, and tables are linked to each other by keys. You talk to them with SQL, a language that has been around for decades and is still used in nearly every tech role, including data and product work.
Relational databases are a good default because they protect your data. They can enforce rules, such as "every payment must belong to a real order", and they support transactions, which make a group of changes happen completely or not at all. For anything involving money, that matters a lot.