By mining, participants add blocks to the blockchain. To do so, they must dedicate computing power to solving a cryptographic puzzle. As an incentive, there is a reward available to whoever proposes a valid block.
It’s expensive to generate a block, but cheap to check if it’s valid. If someone tries to cheat with an invalid block, the network immediately rejects it, and the miner will be unable to recoup the mining costs.
The reward – often labeled the block reward – is made up of two components: fees attached to the transactions and the block subsidy. The block subsidy is the only source of “fresh” bitcoins. With every block mined, it adds a set amount of coins to the total supply.