Employers negotiate group rates with insurance carriers, and typically pay some portion of the premium. COBRA basically allows you to stay on the employer plan even when you are no longer qualified to do so (typically because you no longer work there) to give you time to find a new job or new insurance or whatever. The caveat, however, is that you have to pay the full premium yourself; no more employer contribution. This makes maintaining insurance coverage through COBRA prohibitively expensive for a lot of people.