What depends upon a red wheelbarrow?
Answer: So much.
What is the significance of the glaze of the rain water or the white chickens?
Answer: So much depends upon the reader/observer.
There is no "I" in the poem. There are no capital letters, rhyme or meter. The objects are ordinary. Aside from the reference to agriculture or labor (farming), this scene is of objects with no overt meaning. This scene, sparsely described, is an example of Imagist poetry, the poetic attempt at painting a picture. Williams was attempting a new kind of poetry, perhaps even beyond Imagism. With no discernible style, no punctuation and no cultural or historical references, this is an image, plain and simple; there are just words which stand for things. This might sound ridiculously simplistic, but it goes to the heart of the poem. Words depend on things. We depend on words to communicate things (and ideas).
Using this plain image, the reader focuses on the word/images, the odd line breaks and maybe even the choice of prepositions. Why "red?" Why "a" and not "the" wheelbarrow? Why "chickens"? Regardless of the poet, I'm of the school that the reader's interpretation is the creation of meaning. But even moreso in cases like Imagistic poetry.
To answer your question, my (and this is only one interpretation) impression of the poem is about nature and industry, represented by the chickens/rain and wheelbarrow respectively. The wheelbarrow implies human industry but there is no human in the poem. The chickens and rain (plant growth, drinking water, life, etc.) represent nature; chickens a source of food. So a basic interpretation is that so much depends upon nature and industry for survival. But to be honest, I would call this a dumbed down impression. The significance of this poem is that so much depends upon words which depend upon things; for good or bad. As these are simplistic things, another interpretation could be that this is a criticism on the accumulation of unnecessary things. Instead, Williams offers a focus on more simplistic objects and how they can convey or prompt the reader/observer to form as much meaning and introspection with his/her own imagination --- perhaps even more than a descriptive, stylistic poem might convey.
In this sense, why "chickens," why "a red wheelbarrow," why "glazed with rain" is the whole point - focus on objects themselves. So much depends upon imagination via words via objects.
In Spring and All, Williams writes: “To refine, to clarify, to intensify that eternal moment in which we alone live there is but a single force—the imagination.”