I frequently see advice about how to learn chess that include links to databases, or how to set them up, or a question here about Analysis where the accepted answer says to just add some stuff to your database.

But **what is that database, really?** I never saw an explanation for that.

And **how would a beginner use a database?** Why does everyone talk about databases but never explain what they are in first place?

I assume they are important, since seemingly there are lots of companies that get lots of money solely by selling chess databases...