A replaced transaction is a transaction in the cryptocurrency world that has been replaced by a transaction with a higher fee, or that has been canceled. When someone sends a cryptocurrency transaction, the network will add it to the pool of pending transactions, or mempool.
The miners run their nodes and process these mempool transactions according to their fees as long as the cryptocurrency transaction is valid. However, if the sender of the original transaction wants to cancel the transaction or replace it with one that has a higher fee, they can do this using a replace-by-fee (RBF) feature.
Generally, miners will prioritize the highest-fee transactions to process first, meaning that the higher fee transactions usually get processed first and the original transaction will be left in the mempool until it ages and expires. This mechanism is known as a replaced transaction.
Replaced transactions are a useful tool when it comes to recovering funds stuck in mempools due to low fees or slow processing times. By using RBF, senders can increase their chances of getting their transactions processed by miners quickly by submitting higher fee transactions.
However, RBF should only be used when absolutely necessary, as some cryptocurrencies may not support this feature and senders may still experience delays even when using RBF.