Block reward is the reward given for work performed by a miner for hashing an algorithm in a Proof of Work blockchain or cryptocurrency or for performing computational work on the blockchain.
Block rewards are given to the miner or group of miners who discover the next block in a blockchain.
In Bitcoin the block reward currently is at 12.5 BTC per block with a reduction (halving) down to 6.25 BTC per block coming up in 2020. In Ethereum the block reward is 3 ETH per block with a decrease to 2 ETH per block being implemented upon EIP 1234 going live.
Block rewards are also implemented in in Proof of Stake systems where stakers replace miners and stakers who get to verify or make new blocks are awarded cryptocurrency or tokens for their work.