Until a transaction confirms it is just a proposal in the mempool, and RBF formalizes updating that proposal: the replacement spends the same coins with a better fee, and nodes drop the old version. Most modern wallets expose it as a "bump fee" button.
The receiving side has a mirror tool — CPFP — and both are part of the never-get-stuck toolkit in how long transactions take. One practical note: RBF is why zero-confirmation payments should never be treated as final.