This is the insurance that covers the cost of your hospital or doctor's treatment. This may include treatment in a private ward or an appointment in the NHS beforehand. Some plans also allow you to apply if you cannot be accepted by the NHS within a certain time frame. Other plans may charge a bit more and have nothing to do with NHS waiting times.
You can get a medical check-up at the start and insurance will cover you from the start (so you know what's covered and what isn't) or you can opt-out of a medical check-up at the start (but conditions that occurred two years before you took out insurance aren't covered) and often there is no coverage if they reoccur within five years after being paid out of the plan. Contributions are usually verified annually.