What is a phenomenon characterized by poorly planned development on the edges of cities and towns.

They're most likely referring to urban sprawl, which is usually associated with rapid, less thoughtfully planned development.

It's usually thought of in a negative light, because the neighborhoods that develop lack the amenities, infrastructure, and/or identity of traditional neighborhoods. And because they're on the edges of cities, everyone usually relies on cars, which leads to traffic jams, pollution, and a lack of sense of community.