Rear Admiral was a naval rank, usually the lowest level of Admiral in a fleet, and the counterpart to the Army rank of Major General. The next rank up in seniority was Vice Admiral, while the rank below was often Commodore, but in the Imperial Navy, apparently Commander. There is also some evidence for a semi-formal subordinate grade of Brevet-Admirals, necessitated by the vast expansion of military command responsibilities under the Galactic Empire, and later during the Yuuzhan Vong invasion.
Almost the only Rear-Admiral to be explicitly identified as such in the sources is the Imperial officer Michael Unther, who was promoted to Rear Admiral on retiring from active duty: somewhat bizarrely, he had previously been the commanding Admiral of a Sector Fleet, a statement which is hard to explain unless this had been a brevet posting. An analysis of rank insignia may help identifiy other Rear-Admirals, however - suggesting for example that Delak Krennel held this rank during his mission to Axxila in 4 ABY.
Due to the lack of evidence, it is hard to say with confidence what sort of duties Rear Admirals typically undertook in the navies of the Old Republic, Galactic Empire, New Republic and Galactic Alliance. It can be suggested, somewhat tentatively, that officers of this rank may have commanded some individual Star Destroyers, or served as the flag officers for local squadrons and as the commodores of Systems Forces. But given that Admiral Unther was actually promoted to Rear-Admiral from a Sector Fleet command, it is not inconcievable that some Rear Admirals held the senior positions of Fleet Admiral and High Admiral within Sector Groups.