Short answer: In an HMO you pick a primary care physician who coordinates your care and must issue a referral before the plan will cover a visit to a specialist.
With an HMO, your primary care physician (PCP) is the hub of your care. For anything beyond routine primary care, the PCP issues a referral that authorizes the plan to cover the specialist visit; without it, the specialist care generally isn’t covered. The trade-off for this extra step is lower premiums and predictable copays. PPO and EPO plans skip the referral requirement.