You can specify your Meta Content for a Category when editing your Categories.

YourDomain.com > Modules > Ecommerce: Products (mX) > Categories

See Screenshot:

The Page Title by Default (if Custom is not selected) uses the Category name_SKU.  Page Titles are important for SEO - NOTE:  THis is different than the page url.

Meta Description By default uses the Description text used in your Category description.  You can customize it by selecting "Custom" and writing something different there.

The Meta Description is what a Search Engine uses to briefly describe your Category. (Most of the time - but not always.  Sometimes the SE shows content from the page - usually to show you where it found the text that you searched for)

Meta Keywords Are keywords search engines use to find your items.  By default this area is populated by the Category name only.  It is a good idea to add more keywords that are relevant to your items. (things that people might use to describe your items when searching for them.)