。
She runs a dental practice.
Literal
She [topic-は] dentistry [object-を] opening-business is.
歯科医 (しかい) literally combines 歯科 ('dentistry, dental department') and 医 ('doctor') — 'dentist.' The closely related word 歯科医院 means 'dental clinic.' Note the sentence uses を rather than では, framing 'dentistry' as the object of opening. 開業する ('to open a practice, to start a business') is the standard verb for setting up an independent professional practice — used for doctors, dentists, lawyers, and similar self-employed professionals. The resultative ~ている marks the ongoing state of running the practice.