Replace by Fee (RBF) is a Bitcoin protocol tool developed by famous blockchain developer Peter Todd that allows one version of an unconfirmed transaction to be replaced with another transaction that pays a higher transaction fee. Via RBF, unconfirmed transactions are stored in the mempool, which is where valid transactions wait to be confirmed by the Bitcoin network. From there, these stored transactions can then be replaced with another transaction if the sender of that transaction offers to pay a higher transaction fee. As a result, while RBF can help reduce network congestion and help prioritize transactions in accordance with senders' willingness to pay, the mechanism is somewhat controversial because it alters the immutability of the involved transactions.
