In a PoS system, validators are given the right to forge a block based on the coin age of their stake. Coin age is determined by how many idle days old the sake is, multiplied by the amount of coin at stake. If the validators stake is idle for 30 days or more, they may be selected to forge the next block. They bigger and more idle the validators stake is, the better the chance of being selected (versus in a PoW system, where your hash power is the determining factor). Once a stake forges a block, the coin age gets reset back to 0 (and must wait 30 days for the chance to forge another block. Coin age has age-cap of 90 days to help level the playing field for other validators.