According to the enotes Study Guide on "The Red Wheelbarrow," the theme of the poem is as follows:
Themes and Meanings
What “depends upon” a red wheelbarrow, white chickens, and rain? The reader is aware of the usefulness—in the case of rain, the necessity—of these things in the external world. The things referred to in the poem are also particular instances of types and classes of things—the wheelbarrow being a machine, for example, on which life also depends. Furthermore, sensations, feelings, emotions, thoughts, and ideas depend on such things. As the poet expresses it in his poem “A Sort of a Song,” “No ideas/ but in things.” The faculty of the mind that has ideas is the imagination. “The Red Wheelbarrow” is about the relationship between the imagination and reality.
The poem, then, is about how human imagination interprets what the senses relate about reality to the human mind.
So much depends upon the "so much," if you will. Whatever associations one finds in the images the poem presents, are dependent upon one's imagination. It is the imagination that sees and finds meaning in objects, glaze (the sun must now be out following a rain) and water, and chickens.
Human minds interpret what they experience through their senses and find meaning, and, according to Williams, it is the imagination that interprets and finds meaning. The other commentators above, for instance, have done just that in interpreting the poem.
I just read a 3,000-word essay about a 124-word Beatles song. While I was reading, not even half-way through, I thought: This is way too much to say about such a simple, little song. The remaining Beatles, if the were to read the essay, would have a great laugh.
And one must believe that William Carlos Williams must have had quite a laugh out of all the words used to discuss his little poem about an image of a wheelbarrow and some chickens. Indeed, the poem was originally printed, without a title, in a 1923 Williams poetry anthology called Spring and All. Look how simple the poem is without the first four words:
a red wheel
glazed with rain
beside the white
It's a picture of contrasts of colors and shapes and textures and things. And what do the first four words say?
so much depends
I don't know... you tell me what depends on that shiny wheelbarrow and those white chickens?
Maybe what the poet is saying is this: stop looking for themes in poetry (and in life). Just look.
The theme would also be life, re-birth, second chances, and the importance of the cycle of life upon everything.
Each verse conveys an image of re-birth, cycles, and entanglement with the universe as a whole.
The life cycle and re-birth are notable in the red wheelbarrow's glaze of "rain water" because it evokes the image of the never ending water cycle that never ends and has been in our planet forever.
It is also represented in the barn and the chickens since another commonly known life cycle is that of the egg/chicken/hen.
The red wheelbarrow itself is the center of it all, and so much depends on it, since it serves in many capacities to help us carry, drag, take, commute..and it is an reflection of ourselves, and how life touches us daily, and we are so indebted to that chance, and to make the best of our time with the efficacy of that red wheelbarrow.so much depends
upona red wheel
barrowglazed with rain
waterbeside the white
This depends upon (ha ha) your point of view.
To me, this poem is just about how simple, everyday things are important. It is about how our happiness depends on appreciating dull things like chickens and wheelbarrows that are out in the rain. In other words, we need to find the joy in our everyday life. Our happiness depends on that.
Scholars take a much more serious view of the poem. One explanation, for example, argues that Williams is trying to talk about the relationship between imagination and reality. That theory believes he is saying that all things are really created by our imaginations.
But poetry is something that everyone can interpret for themselves, so I'm sticking with my idea, which I think is way more understandable.