ELI5 the Ethereum Merge

Although the Ethereum network currently operates through a proof-of-work (PoW) consensus mechanism (similar to Bitcoin), a transition to proof-of-stake consensus has been planned since its initial release in 2015. Proof-of-stake (PoS) is the underlying mechanism that authorizes validators with sufficient economic value at stake. 

PoS validators are chosen at random to create blocks and are responsible for checking and confirming blocks they don't create. A validator's stake is used as a way to create economic incentives that align good validator behavior with the performance of the network. Validators can be penalized for failing to validate or have their entire stake slashed for engaging in deliberate collusion.