Let's say you have a working class community. The people who live there might not be rich, but they can take pride in where they live, develop a local culture, and make it a quality place to reside and raise a family. This is particular true with minority communities, who in the past had difficulty find other places to live.
Through their hard world, the people who live there have made things to nice, richer people start moving in. They start to dilute the culture, making it less diverse and more like any other suburb. They also increase the property values, which means new working class people can't move in, and the existing ones have to pay higher property tax. Eventually, the original people who build the community move out, and what made the community special is lost. Sometimes, the rich people will move out after that, and without a critical mass of working class people left, the community falls into disrepair, and is ruined.
Complicated topics explained, simply.