Medicare Part B medical insurance "helps cover medically-necessary services like doctors' services, outpatient care, durable medical equipment, home health services, and other medical services"(Medicare.gov). When you become eligible for Medicare Part A, you will have seven months to sign up for Part B. If you do not sign up in this period, then you will have to wait until the next open enrollment period -January 1st through March 31st.
It is important to sign up for Part B on time, because if you are uninsured for 12 months or more, your Medicare monthly premiums will be higher. You can learn more about Medicare in the "Medicare & You" online handbook here.