Proof-of-stake (PoS) is a method by which a blockchain of a cyber currency aims to reach a distributed consensus.
While Proof-of-Work (PoW) requires users to repeatedly run hash algorithms by adding a random alphanumeric string to the block data until the overall fingerprint is below a given threshold, Proof-of-Stake requires the user to prove possession of a certain amount of cybercurrency (their "stake") in order to claim to validate additional blocks in the blockchain and collect the reward. (BitcoinDotFr)