A block reward is the amount of cryptocurrency a miner receives for successfully mining a new block, that is, once the miner solves the mathematical puzzle required to add a new block to the chain. This reward system is fundamental to all proof-of-work blockchains and is not unique to Bitcoin.
Bitcoin is designed to have a declining reward system so as to keep Bitcoin prices rising and prevent inflation. Since Bitcoin has a limited supply, eventually there will be no new coins to mine and therefore no mining reward. In the early days of Bitcoin, each miner was awarded 50 Bitcoins for each block they mined, an amount that would be astronomical at this point. Every time 210,000 blocks are added to the Bitcoin blockchain the block reward is cut in half. The last time this happened was in February 2018. Since then, miners have been awarded 12.5 Bitcoins per block mined. Once the block rewards have been halved 64 times, the reward for block mining will finally reach zero, and no new Bitcoins will be produced. The total number of Bitcoins in circulation at this endpoint will be 21 million. This is currently expected to happen in 2040. Once there are no more blocks to mine, and hence no more block rewards, miners will still be entitled to transaction fees.