The block reward is what a miner earns for adding a valid block to the blockchain. It has two parts: the block subsidy and the transaction fees.
Part 1: the subsidy
The subsidy is newly created bitcoin. It is the only way new coins come into existence. It started at 50 BTC per block in 2009 and is cut in half every 210,000 blocks by the halving. Since April 2024 it is 3.125 BTC.
Part 2: fees
Every transaction can pay a fee, which is the difference between its inputs and outputs. The miner who includes it in a block collects that fee. When the mempool is crowded, users bid higher fees to get confirmed sooner, and miners usually pick the transactions that pay the most per virtual byte.
How the miner gets paid
The first transaction in every block is special. It is called the coinbase transaction (not to be confused with the exchange). It has no real inputs and pays the subsidy plus all fees to an address chosen by the miner. Nodes check that this payment is not larger than allowed. If a miner tries to pay itself even one satoshi too much, the whole block is rejected. Coinbase outputs also cannot be spent until they are 100 blocks deep (coinbase maturity), which protects against chain reorganizations.
Why it matters for security
The reward is the incentive that makes proof of work work. Miners spend real money on hardware and electricity because honest blocks pay. Attacking the chain would cost the same resources and could ruin the value of the coins they earn.
The long-term question
As the subsidy shrinks toward zero around 2140, fees must carry the security budget. Whether fee income will be enough is one of Bitcoin's most debated long-term questions.