The explanation was дана in the most Последнее special episode, called The день Of The Doctor (I think. All of these "...of the Doctor" titles can get me muddled). He had run out of regenerations and was dying. Clara spoke through a slit in time and Космос to Gallifrey asking the other timelords to help him. They sent him spare regenerations. Most Фаны are assuming he's got another full set now.
