Eth2 refers to a set of interconnected upgrades that will make Ethereum more scalable, more secure, and more sustainable.
With this upgrade, Ethereum will move from PoW to PoS and introduce sharding. The Ethereum network will significantly reduce energy requirements, allow more transactions to be processed simultaneously, and improve network security.
You can read the official Ethereum 2.0 introduction to learn more about Eth 2.0.