Tag Credits can be used to buy cosmetic chat tags through the in-game Tag Shop. 

Access the Tag Shop with /tshop, select a category, and click a tag to purchase it. You will receive a tag voucher, which you can right-click in your hand to bind the tag to your account. Once you have the tag, use /tags to select it.

The cost of tags ranges from 1 Tag Credit for common tags, to 6 Tag Credits for Dynamic Tags, which update live with your in-game statistics.