Short answer: In an HMO, your PCP is the coordinator of your care; you use them for routine needs, and they must issue a referral before the plan will cover a specialist.
Within an HMO, the primary care physician (PCP) is the hub of your care. You see the PCP for routine and preventive needs, and for anything beyond that they issue a referral that authorizes the plan to cover specialist care. Without that referral, specialist services generally aren’t covered. This gatekeeping is what helps HMOs keep premiums and copays low.