The block subsidy is the means by which all bitcoins available for exchange on the network are issued. The mechanism is designed such that the Miners who build the network infrastructure receive these rewards as a subsidy to fund their operations.
The block subsidy originally awarded each successful block discovery, 50 bitcoins, however the rate halves once every 210,000 blocks (approximately every 4 years). The block subsidy ceases when the reward halves below the value of 1 Satoshi, an event which is estimated to take place in around 2140.
This chart shows both the rate of subsidy over time and the overall issuance of Bitcoins into the system. Used with permission, courtesy of Metanet.icu user 'mpapec'.