After the city is destroyed, Montag is in shock; he realizes that he has no memories of his wife Mildred doing anything. He wants to have memories of their love and their happy times together, but instead he just remembers her sitting and doing nothing. Granger tries to comfort him with a metaphor:
"Everyone must leave something behind when he dies, my grandfather said... Something your hand touched some way so your soul has somewhere to go when you die... The difference between the man who just cuts lawns and a real gardener is in the touching, he said. The lawn-cutter might just as well not have been there at all; the gardener will be there a lifetime."
(Bradbury, Fahrenheit 451, Google Books)
His point is that since people die, people must leave a part of themselves for future generations to see and experience. Books are an example of this; people write books so later generations can read them and see what they thought, what they believed, and what happened during their lives. The gardener uses his knowledge to care for plants, making them better after his passing; the lawn-cutter just cuts, knowing the grass will grow again. Montag is sad because he believes he has left nothing behind; Granger assures him that he himself has a lot of time to create something to leave.