They are your parent's parents. They have been around for a long time and they deserve extra respect. That idea is a typical response to a great grandparent, and you can have that status if you do not do anything to prove your status otherwise. Grandparents can be the most important people in a child's life. Grandparents can be the people children run to if the relationship between them and their parents is rocky.
Grandparents can be the shining light in a storm, the storm being life in general.
If you follow the rules mentioned above, you could be that shining light, also. Being a grandparent can be the most important job in a person's life. You can make such a difference in a young person's life. That is why you have to be a great grandparent, and not a lousy one. The grandparents I have had in my life have made a great impact on me.