What are miners?
Under PoW consensus, actors known as 'miners' carry the responsibility of verifying the transactions, creating the blocks, and maintaining the chain. In exchange, these miners are given a reward (in ETH) each time their node is the first to finalize, or mine, a new block; this also incentivizes miners having good-quality equipment and connection speeds, which in turn helps the network.
However, there exists the possibility for enough miners to band together--at least 51% of the network--and subvert control of the network to their own ends, rewriting the transaction history, stealing tokens, etc. In order to prevent this and other security problems, the mining is made intentionally difficult, that is, computationally complex (often called "expensive") in order to make it virtually impossible for any rogue actor(s) to carry out such an attack.
This design--made to keep the network safe--has side effects, in particular environmental and real-world economic ones; the economic model on the blockchain is also inefficient. The constant incentive to mine blocks faster means that miners have a real motivation to buy new computers, even specialized ones that do nothing other than mine on blockchains. And big, powerful computers use a lot of electricity, and generate a lot of heat. This environmentally-unfriendly, inefficient arms race has become increasingly difficult to justify, on a number of points. Enter PoS consensus: instead of miners, validators are the actors ensuring transaction validity and network integrity. In place of costly number-crunching as a security measure, each validator must have staked 32 ETH; that is, deposited it in a smart contract, a kind of computer program that lives on the Ethereum blockchain, with the promise that they will operate their validator according to the rules. If they act in bad faith, or try to subvert or attack the network, or just don't maintain enough connectivity, their staked currency will be slashed, or taken from them. If they do what they're supposed to do, maintain connectivity and confirm transactions, they will be rewarded with ETH, the same as miners. For more on PoS, see here.