Ethereum's Proof of Stake (PoS) is a consensus mechanism that serves as an alternative to the previous Proof of Work (PoW) system. PoS was introduced in Ethereum to address issues such as high energy consumption and centralization that are associated with PoW.
In a PoS system, validators are required to put up a stake (or collateral) of ETH in order to participate in the network and validate transactions. The validators are then selected to create new blocks and validate transactions based on the amount of ETH they have staked. Validators are incentivized to act honestly because if they validate fraudulent transactions or act maliciously, they can lose their rewards - known as "slashing".
Ethereum's PoS system is called Ethereum 2.0 or ETH2, which is a major upgrade to the Ethereum network. Validators in ETH2 are required to stake a minimum of 32 ETH to participate in the network, where they are then randomly selected to create a block and validate transactions based on their stake.