When it comes to average money the countries simply print more cash. Now when we talk about bitcoins the money needs to be mined. Minings is done by hundreds of thousands of computers all over the world that need to solve puzzles faster (competing with each other), the fastest ones get the prize (bitcoins).
Mining with a single computer will not be good, that is why having hundreds of computers used at the same time is necessary. After time, the puzzles get harder, that way the computers spend more and more time to be able to mine the money. Otherwise, if things were too easy, there would be a lot of bitcoins in the market, what is not the whole point of the digital currency.
That is the basic idea.