In the field of SEO, keywords are the words which are searched for by people on Google (or other search engines).
It's important for an online merchant to know which keywords their target market use in order to find products online. By matching your store's language to users' language, your website will rank better on search engines.
That means more traffic and more conversions for you. Calling your category page "Yoga pants" instead of "Workout trousers" may bring a world of difference to your traffic.