A referral usually means someone at the firm submits or endorses your application through an internal process. The exact mechanics vary by firm, but the broad idea is that your candidacy gets additional context from someone already inside the organization.
What it does not mean is a guaranteed interview or a bypass around the normal evaluation steps. The application still needs to be strong enough to stand on its own.
So it is best understood as a signal and a visibility boost, not as a guaranteed outcome. You may also find it helpful to read how consulting referrals actually work in recruiting if you want a more structured way to practice this.