Since its beginning, the Ethereum network has been using the Proof-of-Work consensus mechanism. As we know, PoW consumes a lot of energy. Now the Ethereum network is moving to Proof-of-Stake consensus mechanism.
The merge is when the switch from PoW to PoS is going to happen.
Since the December of 2020, there has been a PoS network running in parallel, called the beacon chain. The existing PoW network (main net) and the beacon chain would merge into one and the block production would happen via PoS mechanism. That’s the merge.
All accounts, balances, and smart contracts history of the current Ethereum network would remain intact and only the block production mechanism would change. Before merge — PoW blocks. After merge — PoS blocks.
Read more about the merge here — https://ethereum.org/en/upgrades/merge/