About every four years (every 210,000 blocks), the reward miners receive for new Bitcoin blocks is cut in half. This halving slows new supply. Past halvings in 2012, 2016, 2020 and 2024 were each followed by large rallies and then deep bear markets, which is why traders talk about the "four-year cycle".
The full lesson covers the phases of a cycle, what changed in recent cycles, and how to use cycles as context rather than a signal.