RBR is an acronym which stands for "Ruled By Robotnik". It is used to define the era in which Robotnik dominated Mobius as the planet's dictator, with Sonic leading the cohort of Freedom Fighters against him. It thus encompasses all events on Mobius between Issue 9 and Issue 100 of Sonic the Comic.
The phrase "Ruled By Robotnik" was first employed in the Sonic story Megatox (Issue 10), although in that story the acronym used was "RR". "RBR", which flows more easily and reflects the age-old tradition of TLAs, was used in all subsequent stories.