The explanation was 给 in the most 最近的 special episode, called The 日 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 太空 to Gallifrey asking the other timelords to help him. They sent him spare regenerations. Most 粉丝 are assuming he's got another full set now.
