You know how when you play hide-and-seek, you need to be really quiet so the seeker can't hear where you are hiding? Well, when doctors and insurance companies make deals with each other, sometimes they include something called a "silent PPO." This is like a secret agreement that the doctor and the insurance company keep quiet about.
So, imagine you're a doctor and you agree to treat patients who have insurance with Company A. But then, without telling you, Company A gives your information to Company B. Company B then tells all their patients that they can come to you for treatment, even though you didn't agree to treat patients with their insurance. This means you might have lots of patients with Company B's insurance, but you're not getting paid as much for their treatment because you didn't agree to that price.
Basically, a silent PPO is when a company uses trickery to get doctors to accept lower payments, even though they didn't agree to it. It's not fair, but some companies still try to do it.