Mining consists of specialized computers competing to solve a complex mathematical problem (proof of work); whoever solves it first validates the next block of transactions and receives newly created coins plus the fees from those transactions as a reward.
Why it keeps the network secure
Altering the history of a mined blockchain would require redoing that calculation for the tampered block and every block after it, faster than the rest of the network combined — something that's economically unfeasible on large networks like Bitcoin (a "51% attack").
Energy cost
Bitcoin mining consumes an amount of energy comparable to that of entire countries, which has been the subject of debate and one of the reasons other blockchains, such as Ethereum, migrated to the staking mechanism (proof of stake), which is far less energy-intensive.
The reward shrinks over time
In Bitcoin, the block reward is cut in half periodically in an event known as the halving, which means miners' income increasingly depends on transaction fees and less on the issuance of new coins.