In the world of Star Wars, there are a vast number of planets, hierarchies, monarchies and armies. There are so many of these organizations that sometimes the terminology used to describe ranks and officials can get a bit confusing. For example, there are a few characters with the title "Grand Moff" — but what exactly is that?
The most well-known Grand Moff is actually the very first person to earn that rank: Wilhuff Tarkin. During the fall of the Jedi and the rise of the Empire, Tarkin was a favorite of Palpatine's and promoted him from Admiral to Moff. A Moff is the governor of a sector of the galaxy. During the reign of the Empire, several high ranking military officers were granted the title of Moff and the corresponding political power that came with the title.
Eventually, the Empire grew so powerful there was even further room for Tarkin to rise in the ranks, and the Emperor promoted him to Grand Moff in Tarkin, a canon Star Wars novel written by James Luceno. The novel reveals that a Grand Moff is in charge of a collection of sectors called an oversector. This means that a Moff is in charge of a single sector, whereas a Grand Moff is responsible for overseeing several sectors and their Moffs. As such, Grand Moffs have even more power than Moffs and have the right and authority to overrule another Moff. They have the ability to control and command Imperial Military resources and are able to exert their authority in any Empire region.
The title of Grand Moff is a big deal within the structure of the Empire. Grand Moff Tarkin is the highest authority on the Death Star base. His role grants him such control that even Darth Vader is his subordinate. Essentially, only the Emperor has more power than a Grand Moff.