BIP 125, found in BIP-125, formalizes Replace-by-Fee (RBF). It lets a user bump the fee on an unconfirmed transaction by broadcasting a new transaction (with the same inputs) that pays a higher fee. This capability is like telling your taxi driver, “Here’s an extra tip—could we move a bit faster?”

While it can help in times of network congestion, RBF has also sparked debate about zero-confirmation payments. Merchants relying on unconfirmed transactions might find them less secure if those transactions can be replaced. Nevertheless, RBF offers a flexible way to ensure timely confirmations when mempool fees are volatile.