Medicare Part B is Medical Insurance.
Part B is paid for by the monthly premiums of people enrolled and by general funds from the U.S. Treasury. It helps pay for doctors' fees, outpatient hospital visits, and other medical services and supplies that are not covered by Part A.
Generally, people who are over age 65 and getting Social Security automatically qualify for Medicare Parts A and B. So do people who have been getting disability benefits for two years.
In general, you are automatically enrolled in Medicare Part B and must opt out if you do not want it.