The ground-floor seats closest to the stage. (In a theater, "orchestra" can mean the seats, not the musicians.)
In American theaters the main floor of seating is called the orchestra, a leftover from the days when the musicians sat at the front of that level. Orchestra seats are usually the most expensive. The front rows are not always the best; many regulars prefer the middle of the orchestra or the front of the mezzanine, where you can see the whole stage picture.
Also seen as: orchestra level, orchestra section, the orchestra level