Let the snake wait under
and the writing
be of words, slow and quick, sharp
to strike, quiet to wait,
-- through metaphor to reconcile
the people and the stones.
Compose. (No ideas
but in things) Invent!
Saxifrage is my flower that splits
The first half of this poem seems to be about words themselves -- that is "the writing," explained metaphorically with the snake. Williams is expressing his desire to see a new sort of language emerge in poetry, a language that is just as accurate and effective ("sharp to strike"), as well as determined ("quiet to wait, / sleepless") as a serpent hunting its prey.
But furthermore, especially in the second stanza, Williams seems to be referring to a sort of lingual existentialism. "No ideas but in things" is the most famous line from this poem for a reason. With this simple phrase, Williams beautifully reiterates the entire basis of existentialism: there is no essence without there first being existence (that is, there are no ideas of things without the things themselves existing first). This is why "the people and the stones" must be "reconciled" -- not humans reconciled with stones, but humans reconciled with humans, stones reconciled with stones, selves reconciled with selves. Not only the philosophy, but the language of our times as well, has apparently been corrupted by the illusion of an essence. According to Williams, a person can never be just "a person" -- instead, he will always be himself, a singularity, an existence who will always prove more beautiful than the idea of "a person" (that is, any person). Interestingly, Williams understands literature as a way of expressing singularity, a way of illustrating existence itself (by no means a simple "individuality," which always expresses itself in the form of lonely predicates, or in other words the very "ideas" to which Williams refers*), hence his poems' frequent use of detailed of imagery.
The final line is a metaphor that concludes the poem perfectly: his vibrant saxifrage, that is his gorgeous imagery, will subvert and "split" all of the supposed concreteness and coldness of modern language.
*Everything in these parentheses is extremely unclear, but I can clear this up eventually