Unsurprisingly, the Bitcoin Halving is when Bitcoin’s mining reward is split in half. Halvings occur about every four years; four years because the Bitcoin mining algo is set with a target of finding new blocks once every 10 minutes (more or less; on average 9.66 minutes), which would take about 1,409 days to mine the 210,000 blocks required. The having is then triggered every 210,000 blocks. 

In effect, halvings reduce the rate at which crypto is introduced and, as a result, the available amount of new supply. The first reward was 50 bitcoin. The last halving was on May 11, 2020, to 6.25 BTC. The next having is expected to occur in April 2024, when the block reward will fall to 3.125 BTC. 

The process of bitcoin mining was highly mapped through the work of Satoshi Nakamoto, the presumed pseudonymous person(s) who developed Bitcoin. First, the halving was intended to counter any inflationary effects on Bitcoin by lowering the reward amount and reducing the amount of new supply to maintain scarcity. Second, demand for new Bitcoins has historically increased upon each previous halving event. For miners, the halving may result in consolidation in their ranks amongst other miners since the new Bitcoins released into circulation are cut in half and the rewards are too. 

The year 2140 is expected to be the last Bitcoin halving when the proposed limit of 21 million bitcoin is presumably reached and no more coins will be created. Why 21 mil? An “educated guess,” according to Satoshi whose goal was to make the number one that would eventually make prices denominated in Bitcoin comparable to existing currencies. See https://river.com/learn/can-bitcoins-hard-cap-of-21-million-be-changed/#:~:text=Satoshi%20explained%20that%20the%20decision,Bitcoin%20comparable%20to%20existing%20currencies.