Medicare covers roughly 80% of the cost for covered care, the rest is your responsibility. A Supplement plan can assist you paying for the remaining 20%.
Here is a comparison of the various Supplement (Letter) Plans available

Supplement plans have a monthly premium which is in addition to your part B premium.
You should also consider maintaining a PART D Prescription Drug plan
Routine Dental and Vision care is not covered by Medicare, so separate policies are recommended