it means that when you use seeds on dirt ill turn it to grass or on cobblestone = mossy cobble same with stone bricks

Whenever you use seeds on a block like dirt or cobble, it has a chance of making it into grass or mossy cobble. Same goes for stone brick (it will turn into mossy stone brick when successful). The ability is tied to the MCMMO stat of Herbalism, so as you level up in that, you're chance of failure decreases. The message you kept getting was from the ability failing due to a low level in herbalism. Note that even if it fails, it WILL use up your seeds.
More on the Herbalism skill can be found here: http://mcmmo.wikia.com/wiki/Herbalism
And more on the MCMMO skills in general can be found here: http://mcmmo.wikia.com/wiki/McMMO_Wiki

Hope this helped.