The event every 210,000 blocks (roughly four years) when the block subsidy of new bitcoin released in each block gets cut in half.
The Bitcoin network has a hard cap supply of sats. The way sats enter the market is through the block reward. The block reward consists of two parts, the block subsidy and the transaction fees of the transactions verified in that block.
The halving is an event that happens every 210,000 blocks (roughly four years) where the block subsidy is cut in half. This supply schedule and the reduction in the block subsidy every four years is written into the Bitcoin network code and can't be changed.
This is a good thing because it allows all the Bitcoin users to know the supply and supply schedule ahead of time for new sats entering the market. This is the opposite of the current system where politicians and bankers decide on when and how much money supply is created.