Every 10 minutes, the bitcoin network produces new bitcoins to reward the people (miners) who run expensive computers that secure the network and confirm transactions. Every ~4 years, that amount that is produced every 10 minutes is cut in half. When bitcoin launched in 2009, the reward was 50 bitcoin. Then 25. Then 12.5, and now with the 3rd halving, the reward is now 6.25. So every 4 years, the amount that is produced every day is cut in half. This makes bitcoin become more rare every 4 years. Hope this helps.