The goal in this game is to start a successful fashion line. This doesn't mean that you "design" clothes, but rather, the outfits you "create" will be based on what people say to you in the street. Luckily, you don't have to know exactly what they're saying; just make sure to memorize any highlighted Japanese characters.
Beyond this, though, you also have to manage a little boutique in which your clothes will be sold. You can wear your outfits around town to show them off, and hope that will inspire customers to come to your store. Then, if your store becomes successful enough, you can open a new one and face different rivals.
Clearly That's QT is too niche for everyone to enjoy, but if you're into fashion or management sims, it might be worth a look.