Medicare is a federal insurance program for people over the age of 65. It is broken out into two major components. Medicare Part A is designed to pay for expenses you incur in a hospital setting. Medicare Part B is for expenses you incur for medical treatment from a doctor, much like common medical insurance would.
Part A you do not pay for and receive automatically upon turning 65, however for Part B you do need to pay a monthly premium for that. You can elect not to participate in this program, which is essentially group medical insurance.
The important aspect about this for individuals thinking about long term care insurance is that Part A is for hospital stays or periods where you may be confined to a nursing home. Part B pays for other medical expenses.
Here are some questions on Medicare posed by an individual asking about income guidelines.