What is replace-by-fee (RBF)?
A way to replace an unconfirmed payment with a conflicting version that pays a higher fee. Only one can remain in the same valid chain.
Wallets call it Speed up or Bump fee. The replacement spends at least one of the same inputs, so it conflicts with the original. Once one version confirms, the other cannot be added to the same valid chain.
Bitcoin Core 28.0 made full RBF the default, so its mempoolMempoolThe waiting room for Bitcoin transactions that have been sent but aren't in a block yet. permits replacement even when the original did not signal opt-in RBF. Wallet support and other policy rules still apply, and the original may confirm before the replacement.
The replacement has a new transaction ID. Compare its outputs and status; a replacement is not necessarily only a fee increase.
References
- Bitcoin Improvement Proposals, “BIP-125: Opt-in Full Replace-by-Fee Signaling”.
- Bitcoin Core, “Bitcoin Core 28.0 Release Notes”.