How do I add a new tag?
Press the Label button on the navigation bar in the Library & Deck builder page.
This will open a new window where you can add tags that you can later add to cards.
Tags can be useful to help filter an search for your cards. As you can see, I started using tags to separate certain types of cards like removal or mana ramp cards. These can then be used in the Filters option to help filter for certain types of cards in your collection.