The Question
The Question was, according to the Teselecta's computer, "the first question, the oldest question in the universe, hidden in plain sight." It was the organising principle for the religious movement known as the Silence, who believed that "silence will fall when the Question is asked".
More specifically, 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, a question will be asked — a question that must never ever be answered: "Doctor Who?"
The Silence planned to kill the Doctor, whom they believed was the only one who knew the answer to the Question. (TV: Let's Kill Hitler) They intended to do this by turning Melody Pond into a weapon to kill the Doctor. Her first attempt was in 1938 Berlin and then again in 2011 on the beach of Lake Silencio. Although this attempt also failed, the Silence (as well as the rest of the universe) believed it was successful. Unbeknownst to the Silence, the Doctor had used the Teselecta to fake his death at Lake Silencio. (TV: The Impossible Astronaut, Let's Kill Hitler, 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...". Although the Teselecta's databases were unable to identify the Question, Dorium claimed to know it although it is unknown how he came by the knowledge. According to him, the Question was actually "Doctor who?!" (TV: Let's Kill Hitler, The Wedding of River Song)
