what is that database, really?
A chess database is a file or set of files along with a computer program which allows you to store chess games in a way that allows you to easily search through them. The associated computer program should also allow you to easily hook up a chess engine to help you analyse the games.
how would a beginner use a database?
Every time you play a game where you recorded the moves when you get back home open the database and enter your game in the database. Then you can go through it, first using your own brain trying to work out which were the good moves and bad moves and why and second with a chess engine hooked up to see where the engine thinks good moves and bad moves were played.
Then a few weeks or months or years later when you are drawn to play somebody you played before you can go back to your database, look up the game, work through it again and see what you have to do to get a better result this time.
I assume they are important, since seemingly there are lots of companies that get lots of money solely by selling chess database
Well, I don't think there are any companies making money solely by selling chess databases but it is certainly part of their revenue.
Note that you don't need to pay money to get your hands on a chess database. There are lots of free programs out there which will do the job for you and free sources of games if you want to download other people's games.
For instance, this combination will do the job for you:
Scid vs PC for the database management program
Stockfish for the chess engine
Ordinary pgn files for the database files
TWIC for a supply of the latest top games played around the world
A paid for solution like Chessbase will get you a deluxe solution which could be worth it if you are semi pro but if you just want to look at top games then Chessbase make their database available (with cut down functionality) online for free.
If you need help getting started then Watford chess club have put up the following useful web pages: