Replace-By-Fee lets you replace a transaction that is still in the mempool with a new version that spends at least one of the same inputs and pays a higher fee.
Rules
Bitcoin Core accepts a replacement only if it pays a higher absolute fee and a higher feerate than the transactions it replaces, and it must pay for its own relay bandwidth. These rules prevent cheap spam.
Opt-in vs full RBF
BIP125 introduced opt-in signalling: a transaction with an nSequence value below 0xfffffffe on any input says "I can be replaced". Since Bitcoin Core 28.0 (2024), full RBF is the default, meaning nodes accept replacements whether or not the original signalled.
The lesson: an unconfirmed transaction is not final. Wait for a confirmation.