In order to understand Proof of Stake(PoS) it is important to understand Proof of Work(PoW). Both consensus models have the same goal in mind but go about it differently. The goal is to determine who gets to create the next block on the block chain and in turn be rewarded(ROI) for it. PoW uses a cryptographic puzzle which requires immense processing power to determine the winner. PoS uses a combination of randomness, stake age(how long you've held coins in your wallet), and stake weight(how many coins you've held in your wallet) to determine the winner and reward(ROI). In both scenarios there is an investment of accepted external value(power, hardware, purchase of coins, etc.) into the network to be part of that process. The focus of both consensus models is to maintain decentralization and build a trustless network where everyone is incentivized to follow the rules(protocol).