It is a leftist term for what happens when the children productive citizens (usually graduate students living off their parents), attracted by low rents and cheap housing, move into a slum that has traditionally been inhabited by unproductive layabouts (gritty "urban" types who - conincidentally are also living off the graduate students' parents).
Amusingly, the (usually leftist) very first people to "discover' and move into the slum get very upset when other people are inspired by the improvements they made and follow after them to further enhance the neighborhood.