In the story, Vanka is homesick. He is apprenticed to Alyahin, the shoemaker. However, he is deeply unhappy in the latter's employ. This is because Vanka is continually subjected to physical abuse, and he is given very little food. Additionally, the food he does receive is of poor quality.
Vanka writes to his grandfather and begs the latter to take him home. Vanka promises that he will powder or prepare his grandfather's snuff for him. Usually, snuff is inhaled. It is made from pulverizing dried tobacco leaves. So, in offering to "powder" his grandfather's snuff, Vanka is offering to prepare the latter's favorite indulgence in the traditional manner: by crushing dried tobacco leaves until they turn to powder.
Vanka also promises to pray for his grandfather and to perform any odd jobs that need to be done. In addition, Vanka promises to take good care of his grandfather in the latter's old age. He tells his grandfather that he will not let anyone annoy or upset him. Last, but not least, Vanka promises his grandfather that he will pray for his soul upon his death.