Most software engineers, particularly those working with backend applications, will use data stores. But do you understand something more about the application underlying that nice GUI interface that you double click on? How would you write an application like MySQL or MongoDB or Neo4j? Questions like these could lead into general database principles such as ACID; SQL / NoSQL; different types of NoSQL; transactions; logging; file storage; blocking/non-blocking I/O; threading; indexing; sharding; query optimisers etc. As with question two, the interviewer is asking an open-ended question in order to investigate the bounds of your knowledge.