Replace by fee is a proposed mechanism for bitcoin (that has not been adopted) by which 0 confirmation transactions can be "undone" by submitting a transaction with a higher fee.
Technically, this isn't part of the consensus protocol, since 0 confirmation transactions aren't in the blocks, but miners and nodes will ignore replace-by-fee transactions.
Does Monero support replace-by-fee? If I submit a transaction that contradicts another transaction but has a higher fee, which will (probably) be mined?