If the health insurance company has contracted with a health care provider to give discounted prices for its members, then that provider is "in network."
Provider networks offer the most choice and the best price for members.
Because a health insurance company represents a large membership of potential patients, providers will offer price breaks to those members. It's a similar concept to club stores like Sam's Club or Costco. Remember that not all health care providers contract with all insurance companies. Not all health insurance networks are open to all providers, either.
See any provider, but pay more for out-of-network services
With some health insurance plans, you can see any provider, regardless of their network status. You should be aware, though, that your plan may not cover as much of your medical costs billed by out-of-network providers, meaning that you will pay more out-of-pocket than if you had chosen an in-network medical provider.