In essence, that's the percentage of guilds raiding who've completed that achievement. And the toughest achievement in the game right now is The Immortal, which requires you to get 25 people through Naxx without dying once. Only about 1.5% of guilds raiding have finished that one. After that, it's Heroic: You Don't Have an Eternity (take out Malygos in six minutes), and Heroic: Shocking! (bring down Thaddius without crossing charges). The hardest 10-man achievement is the non-Heroic version of the Malygos timed achievement -- The Undying actually isn't too bad, with almost 20% of guilds having attained it.
More after the break, including information on the easiest raiding achievements.