A Truth Field was an energy field within which it was impossible to tell a lie. The Doctor had encountered Truth Fields before and described them as "quaint". (TV: The Time of the Doctor)
When the Time Lords, trapped in a pocket universe, following the conclusion of the Last Great Time War detected the time field. Gambling that this was a way back to N-Space, (PROSE: A Brief History of Time Lords) the Time Lords seized control of the crack and broadcasted the Question through it, also transmitting the Truth Field to ensure that the only person who could answer correctly would be the Doctor, ensuring that they had found the correct universe and that it was safe to return.
When the Eleventh Doctor and Clara Oswald arrived in the town of Christmas on Trenzalore, where the Time Lords were transmitting from, they discovered that the field caused them to reveal truths about themselves, being advised to stop talking until they got used to it. (TV: The Time of the Doctor)
After three centuries fighting the Siege of Trenzalore, the Doctor had learnt how to exploit the field by simply saying partially correct statements, such as tricking a group of Ice Warriors by telling them to call him "Elias" without saying that that was his true name, (PROSE: Let it Snow) or telling a Wooden Cyberman that he had a sent a signal to its weapon to backfire, neglecting to mention that his screwdriver had no effect on wood. After a further six centuries of fighting, the Doctor had learnt to briefly circumvent the field, telling a resident that he had a plan only to then immediately tell Clara that he didn't. (TV: The Time of the Doctor)