Symbolism, Imagery, Allegory
His wife and child led more sheltered lives in homelike apartments under the kitchen, far removed from the risks and dangers of the dreaded house above. But there was a grating in the brick wall of the house, just below the floor level of the kitchen above, through which Arrietty could see the garden—a piece of graveled path and a bank where crocus blooms in spring. (2.2)
Meet Arrietty. Borrower. Closet nature lover.
Or wait. There has to be more to it than that, right?
Maybe the scene where Homily realizes she and her husband need to let Arrietty explore more than just the rooms in her own house will help us figure it out:
"And it'll give her a bit of interest like and stop her hankering."
"Hankering for what?"
"For blue sky and grass and such like." (6.73-75)
A "hankering" is a deeply held desire or yearning. Just like Dorothy in the Wizard of Oz yearns for the world beyond her front yard and over the rainbow, Arrietty longs to escape her sheltered life indoors to breathe the fresh air of the great outdoors.
The narrator basically comes right out and tells us that's how she feels:
Arrietty watched him move away from the step and then she looked about her. Oh, glory! Oh, joy! Oh, freedom! The sunlight, the grasses, the soft, moving air and halfway up on the bank, where it curved round the corner, a flowering cherry tree! (8.28)
Okay, okay, take a chill pill, Arrietty. We get that you're excited, but it's just grass, right?
Well, not really. It's much more than that. Remember, Arrietty has never seen nature up close and personal before, and now she has the freedom to explore it as she likes. That has to feel pretty good. Just imagine having a whole new world handed to you.
Imagine your parents kept you underground your entire life—how would you feel when sunlight first touched your face? Probably pretty rootin' tootin' awesome. And that awesome feeling is exactly what Arrietty is looking for. When she finally finds it, it's clear that the great outdoors are a symbol of freedom and adventure in our little protagonist's mind.