Trenzalore was a planet which hosted a battlefield graveyard and, incidentally, the tomb of the Doctor.
The size of each gravestone of the cemetery was proportionate to the rank of the buried soldier. (TV: The Name of the Doctor)
The Doctor
According to Dorium Maldovar, the Doctor would visit this place at some point in his travels, which would coincide with 'the Fall of the Eleventh'. Maldovar also told the Doctor that at this time and place 'no living creature can speak falsely or fail to answer.' Here, the First Question in the universe would be asked.
The Silence were determined that the Doctor never reach Trenzalore, so that the question would never be answered. It was a creed of the Silence that 'Silence will fall when the Question is asked', or as Maldovar translated it, 'Silence must fall'. (TV: The Wedding of River Song)
The Doctor decided that he had to go to Trenzalore, because he was told his friends would be lost if he didn't go there. There he saw his tomb, which was the TARDIS itself, where he came across the Whisper Men. He met The Great Intelligence once again, in Walter Simeon's body. (TV: The Name of the Doctor).