The Question

The Question was "the first question in the universe, hidden in plain sight." The Silence believed that "On the Fields of Trenzalore, at the fall of the Eleventh, when no creature can speak falsely or fail to answer, the question that must never be answered will be asked."

The Silence planned to kill the Doctor, whom they believed the only one who knew the answer to the Question. Their first attempt was to frame the Doctor for the destruction of the universe by blowing up his TARDIS, causing the Cracks. This made the Alliance lock him up in the Pandorica, but he was released thanks to his companions and his future self. (DW: The Pandorica Opens / The Big Bang) The second attempt was to turn Melody Pond into a weapon to kill the Doctor by Lake Silencio. Both failed, although the Silence were thought to believe the Doctor dead when he allied himself with the Teselecta to fake his death. (DW: The Impossible Astronaut, The Wedding of River Song)

Dorium Maldovar told the Doctor that "silence will fall when the question is asked" was a poor translation. A better translation would be "silence must fall...". The fact that Silence must fall but not necessarily will fall implies that there is a way to avoid the Silence, even if the Question is answered. The fact that River Song knows the answer to the Question, the Doctor's true name, proves that there must be a loophole in the Silence's prophecy. (DW: Forest of the Dead) Although the Teselecta crew, who possessed great knowledge about time, space and the Universe in general, were unable to tell the Doctor what the Question was, Dorium somehow learned it and told the Doctor that the Question was actually "Doctor Who?". (DW: Let's Kill Hitler, The Wedding of River Song)

It is unknown exactly what the Silence mean when they claim "Silence will fall" but considering how desperate they are to prevent the Doctor from answering the Question, the consequences must be catastrophic.