Urban sprawl is what happens when cities stop being densely populated, compact areas and start becoming less dense and much more spread out.
In the past, cities tended to be very densely populated and compact. Decades ago, this was largely true because of the transportation situation. People largely did not have cars and it would have been very hard for many people to travel long distances into cities every day to work. Because of this, cities were dense and compact. People lived close to the places where they worked. Outside the cities were a lot of open, undeveloped areas.
Today, this is no longer the case in most places. The ubiquity of cars and the vast network of roads makes it possible for people to live relatively far from central cities. They can use their cars and the roads to commute in to the cities if they need to. Urban areas are no longer just dense central cities with people living in apartment buildings. Instead, there are such areas, but huge numbers of people live in single-family homes that have sprawled out in the area surrounding the big cities.
This situation where an urban area becomes less densely populated and more spread out is called urban sprawl.