This achievement requires you to earn 220 internal achievements in the game. These are the same as the Steam achievements, except that to unlock the Steam achievement you need to earn the 5th level of the achievement within the game. With 48 achievement categories, there are 240 possible internal achievements in the game. However, the 5 for this particular achievement don't count (otherwise they'd count toward earning themselves).
Thus you need to get 220 of the 235 possible other achievements to get this one, or put another way, you need to unlock almost every Steam achievement (or be level 5 in 1/3 and level 4 in 2/3 of them) in order to win this.