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frontrunning is the process by which an adversary observes txs on the network layer and then acts upon this information by, for instance, issuing a competing tx, with the hope that this tx is mined before a victim's tx.
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strategy example: transaction A is broadcasted with a higher gas price than an already pending transaction B so that A gets mined before B.
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generalized frontrunners are bots that watch the mempool to detect profitable transactions, copy their code, replace addresses with theirs, then submit the modified tx with a higher gas price.
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specialized frontrunners, also known as longtails, are covered here