The Bitcoin halving is the date when the supply of newly issued Bitcoin is halved.
There will only ever be 21 million Bitcoin in circulation. Bitcoin transactions are grouped in blocks. New blocks of transactions are discovered by Bitcoin miners approximately every 10 minutes. The Bitcoin software adjusts the difficulty of finding new blocks every 2,016 blocks or approximately every two weeks. 6 blocks per hour x 24 hours in a day x 14 days = 2,016. After each 2,016 block period the Bitcoin software adjusts to make it easier or harder depending on how much computing power is dedicated to finding new blocks.
When a group of competing miners discover a new block of Bitcoin transactions they are rewarded with newly created Bitcoin. This reward is called the coinbase. When Bitcoin first launched on 3 January, 2009 Bitcoin miners were rewarded with 50 new Bitcoin. After just under four years or every 210,000 blocks the supply of new Bitcoin is halved. There has been three Bitcoin halvings in history so far.
28 November, 2012 block 210,000 was mined:
Supply halved from 50 to 25 newly issued Bitcoin approximately every 10 minutes.
6 July, 2016 block 420,000 was mined:
Supply halved from 25 to 12.5 newly issued Bitcoin approximately every 10 minutes.
11 May, 2020 block 630,000 was mined:
Supply halved from 12.5 to 6.25 newly issued Bitcoin approximately every 10 minutes.
2024 block 840,000 will be mined:
The supply will halve from 6.25 to 3.125 newly issued Bitcoin approximately every 10 minutes.
The price of Bitcoin has traditionally followed a similar trajectory around Bitcoin halvings. Here you can see significant historical events as well as the Price of Bitcoin Over The Past Ten Years. If history is to repeat itself the bull market should reach the all time highs of around $20,000 per Bitcoin by the end of 2020 and continue to rise until it peaks at the end of 2021.