Proof of Stake is a consensus mechanism used in blockchain as an alternative to Proof of Work.
It provides similar security to Bitcoin’s Proof of Work with a much smaller power and environmental footprint.
The basic concept behind proof of stake, is that you prove your interest in the success of the blockchain through owning a stake (usually a cryptocurrency) in the network.
The higher your stake in the network the more likely you will be pseudo-randomly selected as the node to confirm transactions and produce a block.
As the selection is random, but weighted, no single entity can produce the majority of blocks (without owning the majority of the stake).
The produced block is then validated by other nodes on the network, a check that involves confirming only valid transactions occur within the block, that it has been correctly signed by a valid block producer, and all consensus rules are correctly met.
If the node agrees the block is valid they will add it onto their own blockchain and continue distributing the block to other nodes. If they disagree, they will reject the block and wait for a valid block to arrive.
There are many implementations of this basic method but most will follow a similar pattern to this.