The ETH 2.0 Beacon Chain introduces proof-of-stake to Ethereum. This is a new way to keep Ethereum secure and earn ETH in the process.
Staking and becoming a validator is easier than mining (how the network is currently secured). The more people that participate in the network, the more decentralized and safe from attack it will become.
The Beacon Chain currently exists separately to the Ethereum Mainnet but will be connected soon. The plan is to "merge" Mainnet into the proof-of-stake system that's controlled and coordinated by the Beacon Chain.