Montag dreams of these simple, natural foods—milk, an apple, a pear—after he becomes a fugitive, running from the technocratic nightmare of a society where he is being chased on camera by helicopters and a mechanical dog. On a literal level he is hungry and thirsty: after all, it's not often that you torch your boss and flee for your life. He has also spent the night in the loft of a barn, which would naturally bring to mind simple, farm-like foods. Therefore, it's not entirely odd that he fantasizes about the most basic and wholesome of foods, foods that represent the opposite of the overly mechanized world he has just left. With such thoughts in mind, he descends to the bottom level of the barn:
A cool glass of fresh milk, and a few apples and pears laid at the foot of the steps. This was all he wanted now. Some sign that the immense world would accept him and give him the long time needed to think all the things that must be thought. A glass of milk, an apple, a pear.
Montag has finally left the city and entered a pastoral area. The setting symbolizes his movement away from a society crushed and deformed by technology. He has abandoned the false world of walls of television screens and artificial animals. The simplest, most natural of foods align with the new, more natural life he hopes to fashion. Their simplicity symbolizes for Montag a simpler, saner world, where people have time to think. Likewise, the act of hospitality implicit in leaving such food for him, though at this point a fantasy, symbolizes community and social acceptance. In summary, the foods represent Montag's longing for everything he's been denied: a wholesome, non-technocratic society that offers hospitality to this lonely man and a chance for him to slow down, collect himself and think long and hard.
These things would represent to him (if he could get them) the idea that the world was going to give him a break -- give him a chance to remake himself into some sort of self that would be more human.
We are told this just as Montag is getting out of the river after he swims across it to evade the Hound. Montag is hoping that somewhere there will be a farmhouse where he could get those things. If he could find them, they would mean that the world was giving him " the long time needed to think all the things that must be thought."
It seems strange to me that Bradbury would choose those specific things to symbolize that when there are so many other ways to say it. Like, why an apple, a pear and a glass of milk? I don't understand...