You probably already figured it out, but it means "East of the River," which is to say the part of the city that is on the other side of the Anacostia river from the majority of the city. People use it different ways; the city itself uses it to refer to that portion of the city in connection with efforts to improve the lot of the people who live there. But many others use it as a byword for the Part of the City Where White People Don't Live. Not accusing anyone here of that, just FYI. But you should be sensitive to the use of the phrase and think about what it symbolizes when deciding whether to use it.