Shopify uses product tags to control a lot of aspects of the product catalog, including filter management and collections. This help page will show you how to use tags.
To create a new tag, log into Shopify > Products and select any product. Look for the tags area on the right side of the product page.
If the tag already exists, as you start typing it the tag will populate and be highlighted.
You can also click "View All Tags" to see all the existing tags and then to select one.
If the tag you need does not exist; as you enter it you'll be prompted to add it.
Once you add the tag you'll see it in the listing of product tags. Once you save the product, the tag will be added to the library and available for future use.