The halving is an event built into Bitcoin's own protocol that, every 210,000 blocks (roughly every four years), cuts in half the amount of new bitcoins miners receive for validating a block.
Why it exists
It's the mechanism that makes the issuance of new bitcoins decrease over time, until it runs out around the year 2140 when the 21 million unit cap is reached. It's designed to mimic the scarcity of a finite resource, unlike a fiat currency that a central bank can issue without a preset limit.
Effect on price
Historically, Bitcoin halvings have preceded periods of notable price increases, though there's no guarantee that pattern will repeat: the relationship between halving and price combines the reduction in new supply with market expectations and demand cycles that are hard to isolate with certainty.
Not every cryptocurrency has a halving
It's a design feature specific to Bitcoin (and projects that replicate its model), not a universal mechanism across all blockchains.