Proof of stake (PoS) is a cryptocurrency consensus mechanism for processing transactions and creating new blocks on the blockchain.
In this protocol, the blocks are verified by the machines of the coin owners. The owners have to offer collateral in native coins to have a chance to validate the block.
Validators are then randomly selected to mine a block, unlike in Proof of Work (PoW), a competition-based mechanism. This is a safer alternative, as 51% of attacks are no longer possible since the miner's ability to validate a block is randomized.
Proof of Stake was designed to reduce the scalability concerns and environmental sustainability issues (less computing power needed) of the Proof of Work protocol.
Click here to read more on Investopedia.