In DeFi, front-running is when someone cheats by using the open nature of blockchain to "jump ahead of others in line". Given that transaction in blockchain systems are transparent and typically processed in order of gas price, front-runners can strategically place their transactions to capitalize on information they observe in the pending transaction pool (mempool).
NOTE:
Front-running is independent from Primex. We can recommend using private mempools to decrease the risk of it.