What is ETH "the Merge"?

"Merge" is the name of the event in which the Ethereum blockchain transitioned from using a proof-of-work (PoW) consensus mechanism to a proof-of-stake (PoS) consensus mechanism.

It's called a "merge" because it's a merger of two separate blockchains currently running in parallel. The Ethereum mainnet is in the process of “merging” with a special-purpose blockchain called the Beacon Chain.

The Beacon Chain was launched on December 1, 2020 with the sole purpose of “being a proof-of-stake (PoS) blockchain.”

There are no transactions on the Beacon Chain, and no Token or DeFi applications. It is an "empty chain" just to be a blockchain running the PoS consensus mechanism.