“Mining” is a critical feature of Proof of Work blockchains that prevents Sybil attacks. Mining is also how new bitcoins or other native tokens of Proof of Work blockchains enter into circulation.
Bitcoin mining is performed using sophisticated computers that solve extremely computationally difficult math problems. The people who operate these systems are called “miners.”
When a miner uses their processing power to add a new transaction to the blockchain successfully, they receive a reward in the form of a newly-created coin. Mining is not just restricted to Bitcoin; other blockchains use Proof of Work as well, although many are moving away due to high energy consumption.