"Kew Gardens" is not an ordinary "story," if we can understand it as a story at all. To the extent it has a theme, or "activating circumstance," it is a desire to capture the fragmentary nature of reality. The story puts an ordinarily insignificant detail—the oval flower bed, the flowers, and the snail—in the foreground, and so suggests that the complex colors of the light reflected from the flower petals, so minutely described at the beginning of the story, or the decision the snail makes to crawl under the leaf—are as important as any of the conversations the narrator overhears passing the flower bed.
This is not to say that these conversations have no meaning. A few particular themes emerge: there is the casual insensitivity of the man who speaks to his wife about remembering an old girlfriend; the crazed utterances of a old man about "women in black," or a mysterious electrical device installed by the bed on a "neat mahogany stand"; the incomprehensible secret code of two elderly working class women; or the clipped words between the young lovers ("Wherever does one have one's tea?"). In each case, the fragments of conversation we overhear reveal the attitudes of these people towards each other and their obliviousness to the natural world around them—what they actually say is less important. These thoughts of desire, or memory, or death, are incomplete; Woolf seems to suggest that this incompleteness is in itself a kind of "finished state," in that the truest thing an artist can do is be attuned to this incompleteness.
Another way of looking at the story is to see it as a meditation on the limits of language and the story form itself to accurately depict reality. As Woolf remarks about the young couple at the end of the story,
The action and the fact that his hand rested on the top of hers expressed their feelings in a strange way, as these short insignificant words also expressed something, words with short wings for their heavy body of meaning, inadequate to carry them far and thus alighting awkwardly upon the very common objects that surrounded them, and were to their inexperienced touch so massive; but who knows (so they thought as they pressed the parasol into the earth) what precipices aren't concealed in them, or what slopes of ice don't shine in the sun on the other side? Who knows? Who has ever seen this before?
The words whose wings are too short "for their heavy body" are like the snail; they have a snail-like perspective of the "common objects" they land on, which seem "so massive" and unknown ("Who has ever seen this before?"). The comparison underlines Woolf's desire to show the reality of the flower bed and the reality of the humans passing by as equally important and equally fragmentary. Like the shifting colors of the flowers described in the first paragraph, the language one would use to describe such things is equally ephemeral.