Cryptocurrencies (Bitcoin is one of them) are digital currencies with no central government behind them. Because of this, there's no central mint either—instead, coins are created by a process called mining.
Computers mine by doing little bits of math that help to process transactions. This keeps the currency usable! In exchange for doing this work, the computers are rewarded with a small amount of new currency, adding new coins to the system. It's one big happy cycle!