Hi Gail,
Welcome to MyMedicareForum.com!
Medicare only pays 80% so the doctor will bill you for the other 20%. As an example the "medicare approved" fee for a doctor visit may be $100. So Medicare will pay $80 of it and the doctor will bill you for the other $20. The "medicare approved" amount is also called "medicare assignment". If a doctor accepts "medicare assignment" it means he won't charge any more than the fees allowed by Medicare. Again, 20% of that fee is your responsibility.
Most people with original Medicare get a Medicare supplement program to take care of all or most of the 20%.