Proof of Stake (PoS) is a way of achieving consensus by distributing validation of a block of transactions based on the number of tokens held rather than by rewarding miners like Proof of Work (PoW). Consequently, PoS is a way of validating a block that requires far less energy that Proof of Work (PoW). An important key aspect of PoS is including a degree of chance to the selection process to avoid a scenario where the richest users are always selected to validate transactions and consistently reap the rewards or to bias validation in their favour.
Source: What is Proof of Stake? (PoS)