To use a db efficiently you need to have a bit of an idea of what it does inside.
You need to have a bit of an idea, sure.
But in order to give any kind of a meaningful answer to the question, you need to have not just a "bit of" an idea, but in act, a deeply solid and intuitive idea. Either that or -- crib the answer from websites like these, know that it's one of the top 30 questions you're likely to be asked.
Which, very sadly, is exactly what the modern interview process (greatly) incentives people to do, these days.