Gilgul'im neshamot (jewish concept of reincarnation) literally means 'circles of the souls' (around lifes or incarnations to the body). This is deep secret of nature explained in kabbalah. Lowest part of the soul - neshama is always a gilgul, like a default life spark of any living body.
Classic kabbalah work Shaar ha Gilgulim ("Gate of Reincarnations") of Arizal or Isaac Luria, (written by his disciple Chaim Vital), describes these amazing, deep & very complex laws of reincarnation and impregnation ("pregnancy") ibbur of 5 different parts of the soul.
The subject of gilgul neshamot, the reincarnation of souls, is not mentioned explicitly in the Torah.
It is usually thought that gilgul takes place after a person passes from this world, after the death of the body, at which time or soon after the soul transmigrates into another body.
Moshe, for instance, was a gilgul of Hevel (Abel) and Shet (Seth), as his name indicates (the Mem of Moshe stands for Moshe, the Shin stands for Shet, and the Heh stands for Hevel).
Either he earned all his NR"N during the first gilgul before he sinned, as in the first possibility that we have just read; or, he earned only his Nefesh before he sinned in the first gilgul, which is the second possibility that will be explained here.
In the first possibility, he cannot receive all his NR"N in his second gilgul, except through the device of reciting the verse "My Nefesh has desired You at night...", as was explained at the end of the last chapter.
In his first gilgul he rectified the Nefesh that he received at birth, and he never received either his Ruach or Neshama before sinning and dying.