It's the number of hours an average person takes to finish the game doing all possible achievements without to much boosting, DLC included if it has any. The moderators can vote for it and the average vote of the moderators is shown to all users.
If you complete the game below the average time, no worries, as that alone won't get you tagged for cheating. It just means you are faster then the average person we had in mind when we voted. Perhaps you boosted yourself or with friends or guides a bit more or perhaps your skill was just better.
So unobtainable achievements are not counted and if there is new DLC added or achievements changed significantly the votes will have to be recast.
The reason we are showing the time to 100% is so you can see how much hours you are approximately busy with the game so you can perhaps better decide on what game you want to play next.