Maximal Extractable Value is profit a block producer can take by choosing the order of transactions, inserting their own, or dropping others. You send a transaction. It sits in the public mempool. Anyone can see it before it lands in a block.
Front-running is the common case. A searcher sees a large pending trade, copies it with a higher gas fee, executes first, takes the price move, then lets your trade fill at a worse price. A sandwich attack puts a buy before you and a sell after. Liquidation MEV is grabbing the bonus from liquidating an undercollateralized DeFi loan.
Searchers are bots that watch the mempool and fight to land these sequences. On Ethereum the extracted total runs to hundreds of millions of dollars a year. For ordinary users that shows up as worse prices and failed transactions. It is a tax you did not agree to.
Flashbots and similar systems try to make extraction more transparent and less chaotic for the network. They do not make MEV go away. As long as order in a block has value, someone will bid for that order. Flashbots formed to route MEV through auctions instead of chaotic priority-gas bidding. Searchers bid for block space. Builders assemble blocks.
MEV: Sandwich Attack
How bots extract value from your pending transactions.