From which they pay rent or morgage on the building, utilities, malpractice insurance, staff salaries (or wages), uniforms & linens, costs associated supplies and equipment, maybe advertising (listing in the yellow pages), cost of continuing medical education, memberships in professional organizations. Gross is far less important than net.
The business end of medicine is getting more complex and confusing. There are "preferred provider networks" where in exchange for agreeing to a negotiated lower fee, a provider has a steady stream of patients directed to him. This usually means accepting whatever the insurance pays and patients can not be required to make up the difference - they pay only the deductable and co-pay specified by the insurance company. Patients have incentives to go to "preferred providers" so opening up an office that requires cash up front with the patient doing the paperwork to get reimbursement from insurance works, perhaps, in some specialties (plastics) and in some affluent areas (orthopedics in Aspen) but it can otherwise be very limiting.
Along the way, you should have a good med school course in health care financing and residency programs often offer educational programs about the business end of entering a practice.