What is a replace-by-fee transaction?
Replace-by-fee, or RBF, means that you are replacing one version of an unconfirmed transaction with a new version that pays a higher fee. As a result of increasing the fee, your transaction will now become more attractive to the miners and has a higher chance to be confirmed faster.
While we can never guarantee that your transaction will be included in the very next block. You can use RBF inside of Bitcoin-QT (https://bitcoincore.org/en/download), Electrum (https://electrum.org/#download), Samourai Wallet, Green (https://blockstream.com/green/), Blue Wallet (https://bluewallet.io/), and other wallets.
Another thing to keep in mind is that RBF allows your transactions to be batched together when sending. What this means is that if you send two or more Bitcoin transactions in a row, instead of having separate transactions, the transactions will be automatically combined into the same transaction, allowing you to save on fees!