Consensus nodes (miners or validators) traditionally would serve two roles. They build the actual block, then they propose it to other consensus nodes who validate it. Miners “vote” by building on top of the previous block, and validators vote directly on blocks as valid or invalid.
Proposer-Builder Separation (PBS) splits these up – it explicitly creates a new builder role. Specialized builders will put together blocks and bid for proposers (validators) to select their block. This isolates the centralizing forces of MEV to the builder role, away from validators.
MEV-Boost is an early out-of-protocol iteration of PBS. Eventually, PBS will be enshrined into the Ethereum protocol which will run the auction process internally.