Replace-by-Fee (RBF)

Replace-by-fee is the mechanism that lets an unconfirmed transaction be replaced by a new version of itself paying a higher fee. A transaction sitting in the mempool has not happened yet; if the fee attached is too low for current demand, it can wait for hours or be dropped entirely. RBF is the way out: the wallet rebroadcasts the same spend with a raised fee, and miners, preferring the more profitable version, confirm that one instead. Wallets present it as speed up or cancel, where cancel is really a replacement that sends the funds back to the sender. The consequence worth knowing is on the receiving side: an unconfirmed incoming payment flagged as replaceable can still be changed by whoever sent it, which is why merchants and exchanges wait for confirmations rather than acting on a transaction they can see but not rely on.