Bitcoin mining is a decentralized computational process that confirms transactions in a trustful manner when enough computational power is devoted to block and creates or issues new bitcoins in each block.
In other words.. as Bitcoin doesn’t have a central government that decides when to print and distribute money, Bitcoin uses a protocol similar to gold mining..
Bitcoin protocol stipulates that 21 millions Bitcoins will exist someday, and what miners do is bring them into the light using a special software to solve math problems and issued a certain number of bitcoins. As a reward they get some bitcoins and help the network keep stable, safe and secure for all users.
If you want to learn more, here you have all you need to know about Bitcoin mining.