Front running is the practice of placing a transaction in a queue ahead of other transactions, explicitly performed with the knowledge of the other transactions yet to occur. This can be conducted by a miner who knows of pending transactions awaiting validation.
In front running, a miner can move another transaction ahead in position and ensure its validation or confirmation before other transactions that had happened prior but were waiting to be recorded. The miner performs this with complete awareness that their actions can result in higher profits for themselves.
Front running on a blockchain is typically performed when bots can quote higher gas fees to get their transactions to ‘jump the queue’ and be processed faster. It can result in genuine users with lower gas fees experiencing delays in getting their transactions processed in the network.