The Bitcoin halving is an event during which the reward awarded to miners (those involved in creating BTC) is halved.
The only way to create cryptocurrency is mining, i.e. the extraction of the same. The operation is carried out by the aforementioned miners who are rewarded with BTC.
When Bitcoin was created (January 2009) the reward was 50 BTC. Then, on November 28, 2012, the first halving occurred which led to a halving from 50 to 25 BTC. On July 9, 2016, the second halving took place and the reward dropped again to 12.5 BTC.
Thanks to these operations, the new quantity of Bitcoin placed on the market decreases and thus the circulating mass grows at a slower pace. Over the years, this has prevented the rapid achievement of the 21 million threshold, beyond which it will no longer be possible to produce BTC.
The next Bitcoin halving will kick off in a few days, once the 630,000 block is reached. At the following link all the details and things to know regarding the imminent halving.