To unlock this achievement you need to find a theme'd word.
Some games in wordament come with a theme at the top. You can then search for words which might match this theme for an added bonus. You will see a tally in the top left of how many theme words you have unlocked / how many are available.
If you successfully find a theme'd word this achievement will pop up.
If you are really good and find all the theme'd words from one round you will unlock this: