Mining is the process by which network participants (miners) solve cryptographic tasks and earn the right to add a new block of transactions to the Bitcoin blockchain. In return they receive a reward in BTC and transaction fees.
Miners’ devices (ASICs) try billions of combinations per second until they find a solution that meets the network’s current difficulty. Difficulty is adjusted so that blocks are found on average every 10 minutes. The higher the network’s total hashrate, the harder the task and the more power needed for the same share of rewards.
Mining secures the network: to alter block history, an attacker would need to control more than half of the computing power, which is economically unviable.