Lululemon is going through a bad stretch, but the underlying business still shows promise.Lululemon Athletica (LULU 2.48%) has a cultlike following, with stores that are often crowded with customers. That's good news for the business, but what exactly is the business? On the surface, Lululemon sells athletic wear. But when you look at the price tags on its clothing, it becomes clear that the products it sells are luxury items.Here are three things you need to understand before you buy the stock.1. Fashion i ...