Proof of Stake is a way that people can help make sure that the rules of a particular network are followed, without using a lot of energy.
Imagine a neighborhood where everyone has a special rock that they can use to cast a vote on something.
In proof of stake, instead of using energy to solve math problems like in proof of work, people can use their special rocks to vote on the rules of the network. The more rocks a person has, the more say they have in the voting process.
So, instead of using energy to “mine” new transactions like in proof of work, people in a proof of stake system can earn more special rocks by following the rules and helping to make sure that the network is running smoothly.