A feature to allow transactions in the mempool to be replaced by another transaction with a higher mining fee.
1.1 RBF BTC
When the BTC mempool is full it can take a long time to confirm on-chain transactions. This is due to miners selecting which transactions to include in each block based on the transaction fee. In order to get a transaction confirmed quicker it's possible to re-send the transaction with a higher fee, giving that transaction higher mining priority.
1.2 RBF Abuse
If a transaction is spent with RBF enabled, the receiver should wait for at least a few confirmations to be confident it won't be reversed.
Otherwise the transaction can be sent, received, and then those same UTXO can be sent elsewhere with a higher fee, resulting in the first transaction being replaced and the receiver of the initial funds never receiving their money.
* All terms and definitions may update as the Cryptionary improves.