Learn English – What does “A small town girl with big city dreams” mean

The adjective sprawling is used in the expression "sprawling city" because it conveys the idea of something that moves or grows over a large area in a nonordered way:

• The adjective sprawling is good for describing things that extend across a large area. You could describe your school as sprawling if it consists of many buildings spread over several acres.

• It's pretty common to hear a city or its suburbs described as sprawling, since some cities tend to grow that way, spreading their populations out over a large amount of land. You can also describe your brother as sprawling if he likes to sit stretched out on the couch, taking up more than his share of space.

• The verb sprawl is at the root of sprawling, and it comes from an Old English word, spreawlian, "move convulsively." (Vocabulary.com)

Sprawl (n.):

• meaning "straggling expansion of built-up districts into surrounding countryside" is from 1955. (Etymonline)

Note also the expression urban sprawl:

• Urban sprawl or suburban sprawl describes the expansion of human populations away from central urban areas into low-density, monofunctional and usually car-dependent communities, in a process called suburbanization. In addition to describing a particular form of urbanization, the term also relates to the social and environmental consequences associated with this development. (Wikipedia)

to harness means to make use of something that's typically very hard to make use of (our civilization, for example, has harnessed the electrical phenomena to make our lives better). The idea here is that the way the human mind works is still poorly understood even though scientists have been studying it for centuries now trying to understand how it really works.

So, at this point in time, the human mind is still a mystery to us, but we do know, though very little, something about it. For example, we all know that rote repetition aids retention—the longer you repeat or are exposed to something, the easier it becomes to retain it in your memory. That's one fact.

The other fact about the mind is that when sometimes you hear one of those catchy tunes, the lyrics tend to stick momentarily even without long exposure to the song. We all know that happens. So, what they're talking about here is that we can take advantage of this phenomenon and put that to good use—learning foreign languages, for example. Although we still don't understand how the mind works, this method offers us a small window into the human mind so that we can try and improve our ability to learn vocabulary and phrases as well as retain all that information.