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 which pays a higher fee. This mechanism exists to allow 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, the user can raise the fee they pay to confirm their transaction faster.
RBF only works while the transaction is in the mempool. As soon as a transaction enters a block, it cannot be replaced by fee.