What is a block halving event?

As part of Bitcoin's coin issuance, miners are rewarded a certain amount of bitcoins whenever a block is produced (approximately every 10 minutes). When Bitcoin first started, 50 Bitcoins per block were given as a reward to miners. After every 210,000 blocks are mined (approximately every 4 years), the block reward halves and will keep on halving until the block reward per block becomes 0 (approximately by year 2140). As of now, the block reward is 6.25 coins per block and will decrease to 3.125 coins per block post halving.