Short answer: A PPO is a flexible plan with a broad network, no primary care physician or referral requirement, and out-of-network coverage at higher cost, usually in exchange for higher premiums.
A Preferred Provider Organization (PPO) gives members the most freedom of the common plan types. You can see any in-network provider, including specialists, without a referral, and you also have out-of-network benefits at higher cost-sharing. That flexibility, plus typically larger networks, is why PPOs usually carry the highest premiums. They suit people who want broad provider choice or travel often.