If you're stumped, go to an Under Armour store or find a sporting goods store where their clothes are sold. Next, try on the clothes! Use their products and observe what is different about them, compared to their competitors. Pay attention to how the fabrics feel and how the sizes fit. The more you can describe your experience of using their products, the more knowledgeable you will be in the interview. Product knowledge will give the interviewer confidence that you are invested in the company and that you want the job enough to do your research.
"My most favorite product that I have is your HOVR shoe. It's the perfect athletic shoe for any runner, and I have never found a shoe more comfortable, or better built for optimum performance."