Replace by Fee, or RBF, is a method that allows replacing an already transmitted transaction by transmitting another transaction with a higher fee. This only works on transactions before they have been confirmed on the blockchain. This mechanism exists to allow users to respond if the network becomes congested and transaction fees rise unexpectedly. If a user sends a transaction with a low fee, and finds that it is taking too long to confirm, the user can raise the fee they pay to confirm their transaction faster.