MEV (Maximal Extractable Value aka Miner Extractable Value) is the profit that a miner or validator could extract from including transactions in a block, for example, DEX arbitrage trades or liquidations. It’s a major source of revenue for PoW miners.
The Merge doesn’t remove MEV: validators will still be able to profit from building blocks. MEV-Boost by Flashbots will even offer validators profitable blocks from an open marketplace. However, nobody knows yet how validators will act in practice. Some might avoid MEV of out reputation risks (e.g. famous individuals) or legal risks (e.g. exchanges running validators).