Yeah, “Moff” basically means “Governor”, I think. The Grand Moffs are kind of like the Nazi Gauleiters (regional governors) who headed the various provinces of Germany under Hitler’s rule.

IRL the title “Grand Moff” derives from “Grand Mufti” and was originally applied to Tarkin back when he was some sort of priest-like figure (a Friar Tuck?) who sat on the high command of the Alliance in the 1975 SW drafts.

Either way, the job of a Grand Moff/governor is likely unrelated to the military duties of a Grand Admiral – though that doesn’t explain how Tarkin gets to command the Death Star. Possibly it’s because of his personal pull with Palpatine (a factor that also counted for much among the higher echelons of Hitler’s regime).