What does block reward mean?
Miners use devices to find new blocks on the blockchain and to verify transactions. Their reward for using resources to do this is a share of the cryptocurrency.
In the case of Bitcoin, each time a miner finds a new block they are paid 12.5 BTC (as of March 2018). Initially this reward was 50 BTC, but every the discovery of every 210,000 blocks the reward is halved.
This takes about 4 years for each halving, and by 2040 all Bitcoin are expected to have been mined.