Bitcoin transaction fees compensate miners for including your transaction in a block and are separate from the block subsidy. Fees are based on the transaction's data size in bytes, not its monetary value, and rise when the network is busy as users compete for limited block space. Wallets typically estimate an appropriate fee for you, and technologies like SegWit and the Lightning Network help keep everyday costs low. As the block subsidy shrinks over time, fees will become an increasingly important part of miner revenue.