A hieromonk is a monk who is also ordained to the priesthood, and so may celebrate the sacraments for his monastery or parish, as distinct from a monk who is not ordained.
Monk and priest together
The hieromonk unites two states that are in principle separate: he has professed the monastic life and he has also been ordained a priest, so that he may celebrate the liturgy, hear confessions and administer the sacraments, which an unordained monk cannot, while continuing to live under monastic vows. The office exists because a monastic community needs priests among its members to celebrate the sacraments, and because the priesthood is thereby joined to the monastic life in a single person.
The pool of the episcopate
Because the bishops of the Eastern churches are drawn from the celibate clergy, and in practice from the monastic clergy, the hieromonks are the body from which bishops are chosen, so the office is a station on the path to higher office as well as a role in its own right, and a hieromonk of standing may be raised to archimandrite and thence to the episcopate. Entry is by the ordination of a monk to the priesthood. The hieromonk thus represents the monk-priest of the Eastern churches, the member of a monastic community who has also received the priesthood, and the office marks the point at which the monastic and the sacerdotal are joined, supplying the monasteries with their priests and the church with its future bishops.