Every 210,000 blocks mined, or about every four years, the reward given  to Bitcoin miners for processing transactions is cut in half. This cuts  in half the rate at which new Bitcoin is released into circulation. This  is Bitcoin's way of using a synthetic form of inflation that halves  every four years until all Bitcoin is released and is In circulation.

This system will continue until around 2140. At that point, miners will  be rewarded with fees for processing transactions that network users  will pay. These fees ensure that miners still have the incentive to mine  and keep the network going. The idea is that competition for these fees  will cause them to remain low after halvings are finished.