The Beacon Chain was the original proof-of-stake blockchain launched in 2020 as part of Ethereum's transition from proof-of-work consensus to PoS. The Beacon Chain was launched in December 2020, where developers worked on and perfected the PoS consensus logic before deploying it on the Ethereum Mainnet.
The Merge upgrade in September 2022 saw the Ethereum mainnet transition from a PoW consensus to a PoS one. On the day, original Ethereum clients switched off the mining and block propagation activities and handed over the responsibilities to the Beacon Chain, formalizing Ethereum's transition into a proof-of-stake blockchain.
Before The Merge, the Beacon Chain conducted and coordinated a decentralized network of ETH stakers who started validating Ethereum transactions. However, the network did not process transactions or execute smart contracts on its infrastructure until September 2022.
The Beacon Chain introduced the concept of PoS and staking to the Ethereum blockchain. The transition helped Ethereum become far more energy efficient to operate. The staking activities on the now PoS Ethereum network help make the blockchain more secure by promoting higher levels of decentralization.