The form you are being asked to sign is called an Advance Beneficiary Notice (ABN). Medical providers are required to give you an ABN if they know or have reason to believe that Medicare will determine the service to be not medically necessary for you, and so, will not pay.
Ask your doctor why you need to sign this form if he believes you need the services he is giving you. Medicare should pay for most medical services you need except services specifically excluded from coverage.
If the doctor believes you need the services but that Medicare will not pay for them, ask him to write Medicare a letter justifying your need.
If the doctor will not write Medicare a letter justifying the need for services, you may want to find another doctor and not sign the form.
If you sign an ABN, you may be responsible for all charges.
In limited circumstances, you can ask Medicare to tell you whether or not the service will be covered for you before you get it (prior determination). You can only do this when you receive an ABN for certain very expensive, non-urgent services.
Even after you receive the service for which you signed the ABN, you should still ask your doctor to submit a claim to Medicare. If the doctor explains why the service is necessary, Medicare may pay. It should pay for most reasonable and necessary services.