It's not the first proof of stake blockchain, but it's the first that allows hundreds of thousands of nodes to fully participate, while also having high security and rapid finality. This is only possible because of math that was developed three years ago.

https://twitter.com/technocrypto/status/1330150362427387910

In proof of stake, mining equipment and electricity consumption is replaced by the cryptocurrency itself. You put up your currency as a bond, get rewarded more for running the network, and lose your bond if you misbehave.

The other big advance for ETH2 will be sharding, so each node doesn't have to process every transaction. But that's not the part that launched today.