Nobita's grandfather is the father of Nobita's father. When Nobita's father was little, he used to be treated in disciplined manner by Nobita's grandfather.
Nobita's grandfather died before Nobita was born. Nobita's grandfather appears in two or three episodes. Nobita and Doraemon meet him by going to the past using time machine. He appears to love his son (Nobita's father) a lot, but does not seem to express his affection to his son, so that he doesn't get spoiled.