Eth2 is a long-planned major upgrade of the Ethereum network that enables the Ethereum network to have better scalability, security, and sustainability.
Through this upgrade, Ethereum will change from a PoW consensus mechanism to a PoS and later also introduces a sharding mechanism. This will greatly reduce the need for energy of the Ethereum network and enable the network to process more transactions and improve network security.
Eth2 will be carried out in phases and is currently in the start-up phase, called phase 0. You can read the official introduction of Ethereum 2.0 to learn more about Eth2.