I believe that this question is asking about one of the most important and poignant quotes of the entire book. It occurs when Angus Tuck is attempting to explain to Winnie Foster the dangers of living forever. By this point, Winnie knows about the spring, and she is being forced to consider if it is something that she would like drink from. All of the Tucks, other than Jesse, are not encouraging her to do that. Part of Angus's explanation involves what it means to actually be alive. He says that to be alive, death has to be a reality. Without dying, a person can't honestly say that they are living. It would be like someone saying that they are truly happy without ever experiencing sadness. To Angus, his immortality doesn't mean he's living—it means that he is existing like any other non-living, inanimate object, like a rock.
You can't have living without dying. So you can't call it living, what we got. We just are, we just be, like rocks beside the road.
Scientifically speaking, that rock will eventually weather and erode away. There is still the possibility of physical change for the rock. What Angus has is even worse, in his opinion.