Even though Bitcoin's supply is scarce and set to 21million, none of it was available to anybody when it began...so the problem (and question) was, how do we get bitcoin to the world? The answer was allow people to "mine" for it to solve puzzles to find a block which in returns a bitcoin reward (issuance) if you successfully solved it first. At first these rewards were high for every block, in fact so high that a majority of the 21million is already issued within first 15years of bitcoin's life...but after every 4yearperiodl there's a halving, where these rewards to puzzle solvers get cut in half. Basically next week itll be roughly twice as hard (or more) to get newly issued bitcoin via mining.