When an individual purchase a health insurance policy they are placed in a global “community” of all other policyholders of the same age (or within the same age bracket), with the same plan or policy benefits, who have the same coverage area.
The insurer then calculates the average costs of medical treatment for an individual in the group over the course of the year, and then uses this as the basis of the final policy premium. This premium calculation is then applied to all policyholders within the community – meaning that they will all then pay the exact same amount for their policy.
For groups, the community rating for each individual member is added up to arrive at the overall policy premium.