In Bitcoin, RBF stands for Replace-by-Fee. A Bitcoin transaction can be designated as RBF in order to allow the sender to replace this transaction with another similar transaction that pays a higher fee. This mechanism enables users to respond if the network becomes congested and fees rise unexpectedly.
If a user sends a transaction with a low fee and finds that it is taking too long to confirm, they can raise the fee they paid to ensure their transaction is faster.
RBF only works while the transaction is in the Mempool waiting to receive its first confirmation. As soon as a transaction enters a block and receives its first confirmation, it cannot be replaced by fee.
Additionally, when a transaction is first sent, it must specify that it is available to be replaced by fee if the wallet or platform you are using allows it. Not all wallets/platforms have this feature, so it’s best to see if this option is available if you would like to use RBF at some point.