You can tag your decks with extra information such as it's faction name (#izzet, #jund) or the tournament it will be/was used in (#rptq, #fnm). This is useful for finding specific decks using the tag box filters.
To tag a deck, go to the Details page for that deck and click on the '#add tag' box.
Type in the tag and press Enter. If you are adding an existing tag to the deck, type in the first few letters of the tag then select it from the autocomplete drop down menu.
You can add as many tags to a deck as you like.