A Tech-priest is an adept of the Adeptus Mechanicus. They are the members of the Cult Mechanicus, a priesthood which forms a hierarchy of technicians, scientists, and religious leaders. The Tech-priests provide the rest of the Imperium with its technicians and engineers.
The Fabricator-General of Mars is the leader of the Adeptus Mechanicus, and as the Magos Mechanicus is also the head of its Cult Mechanicus. He also invariably holds a position on the High Lords of Terra. His subordinate and second-in-command is the Fabricator Locum.