Can anyone explain what replace-by-fee means?
It's a method that allows replacing an already transmitted transaction by transmitting another transaction with a higher fee. This only works on transactions before they are signed by miners (0-confirmations).
After 6 years, Now I want to tell you what is Replace by Fee :)
This feature was proposed in BIP125 and implemented in Bitcoin Core 0.12.0.
In simple words, by enabling (Replace By Fee) RBF in a transaction, if a transaction doesn't get mined, it can be resent by the sender with a higher mining fee with keeping the input data of the UTXO(Unspent Transaction Output) same as before.