The father said to him: 'As of a great lighted fire one coal only of the size of a firefly may be left, which would not burn much more than this (i.e. very little), thus, my dear son, one part only of the sixteen parts (of you) is left, and therefore with that one part you do not remember the Vedas. Go and eat!

In plain words: His father explains with a picture. When a big fire burns down, only one tiny coal, as small as a firefly, is left. In the same way, only one of Svetaketu's sixteen parts is left, so he cannot remember the Vedas. The father tells him to go and eat.