Medicare is a federal government health insurance program in the United States. Of course, it doesn’t cover everything. Most people buy additional insurance to bridge the gaps. The two main types of coverage that you can buy to help with this are Medicare Advantage (also called Part C) or a Medicare Supplement which is also referred to as Medigap.
If you choose a Medicare Advantage Plan, you've basically left the govt funded program to go with a private insurance company. This works well for some people and for others it does not, depending on your circumstances. It's important to get balanced, unbiased information so you can make an informed choice. In CA, the HICAP program can help you sort through various plans to help you make the choice that's best for you. If you live in another state, you can call the Area Agency on Aging for a referral to their Medicare Counseling Program.