Trenzalore was a planet which hosted a battlefield graveyard and 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)
History
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 was eventually forced to go to Trenzalore when the Great Intelligence kidnapped a number of his friends. The TARDIS resisted landing on the planet as the Doctor visiting his own tomb was likely to create a paradox and stopped in Trenzalore's orbit. The Doctor then turned off the TARDIS anti-gravity systems to force a crash landing on the planet's surface, leaving one of its windows cracked from the impact. There he saw his tomb, which was the TARDIS itself. However, this version of his TARDIS was in the process of "dying". It had become quite dilapidated and inflated to giant proportions due to the breaking of its dimension dampeners, resulting in a "size leak" phenomenon. It was here that the Doctor came across the Whisper Men and the Great Intelligence taking the form of Walter Simeon. The Great Intelligence forced the Doctor to open his tomb, his real name being the key. River Song, having manifested outside of the Library, spoke his name without being heard and allowed the Great Intelligence entrance.
Inside the tomb was not a body, but an entity that contained the Doctor's personal time tunnel from Gallifrey to Trenzalore. The Great Intelligence invaded this tunnel at the cost of its life to interfere with the Doctor's victories, unravelling everything he had done to protect the universe and beyond. Clara Oswald jumped inside the tunnel as well in order to reverse the damage, though it splintered her into thousands of other echoing versions of herself. After she fixed his timeline, the Doctor leapt into the tunnel to rescue the true Clara, where he reunited with her. However, they encountered an unknown incarnation of the Doctor that dishonoured the name of the Doctor in his actions. (TV: The Name of the Doctor)