Translation:In the suburbs
Native (US) English speaker here. The adjective "urban" means "of the inner city" or "in the inner city" or "relating to the inner city." (It can also mean relating to the city more generally.) I would recommend doing a Google image search of "urban" to get an idea of what it means. The "urban population" means the population living in the inner city for example; "urban lifestyle" would mean the way people live in the inner city. The word "urban" once had a general positive meaning, but at least in the United States the meaning can carry some negative feelings as well due to higher homelessness, crime rates, pollution levels, and other things in the inner city.
The word "suburban", from the prefix "sub" ("under", "less (than)") + "urban", does not mean "underneath the city", but instead means something like "of the less urban areas of a city". These areas are known as suburbs (singular suburb). A suburb, rather than being the inner city, is a part of the city that is more or less on the outer parts (hence "외" in "교외"). It may or may not be inside the official city boundaries. (A smaller city adjacent to a larger one may be thought of as a suburb of the larger city although it is technically not correct.) It is characterized by residential neighborhoods with families living in houses and plenty of space between the houses to accommodate yards and lawns, and is often considered to be better living conditions than the inner city. Usually if you live in the suburbs you will have to drive a little further to access many things because everything is a little further apart because it's not the inner city, but everything is still available fairly close by compared to living "out in the country".