This two minute video explains Replace by Fee (RBF), a way to speed up a Bitcoin transaction you have sent: How Replace by Fee (RBF) works.
What the video covers
If you send a Bitcoin transaction with a fee that is too low, miners may not prioritise it. How long it waits depends on the state of the mempool, the pool of queued transactions.
To raise its priority, you can send a replacement transaction with a higher fee, which takes the place of the original. This is called Replace by Fee, or RBF. It is only possible if your wallet had RBF enabled for the original transaction.
In your wallet, select the unconfirmed transaction and increase the fee. Some wallets offer pre-set fees, such as low, medium and high. Others let you set a custom fee.
A block explorer such as mempool.bitaroo.net shows the fees of recent and pending blocks, which helps you choose a fee that gets the transaction confirmed in reasonable time without overpaying.
RBF is for a transaction you sent. If you are waiting to receive one, see How does Child-Pays-For-Parent (CPFP) work?