"The silent wheels roll through the quiet green," happens to be the first line of a sonnet, a poem written in a highly specific form. The word form here matters: a form, by definition, has a formal definition, one which a poem, or anything else with a formal definition, must fit. A Shakespearean sonnet must consist of exactly fourteen lines, divided into three stanzas and a final rhyming couplet. In the stanzas, each alternate line must rhyme: first with third and second with fourth. A line in a sonnet must consist of exactly ten syllables, or beats, with alternate strong and weak stresses, and each pair of beats must begin with the weaker beat. Like the drumming of Indigenous North Americas, this poetry mimics the beat of a human heart.
Many other formal definitions exist: computer languages have extremely specific formal definitions, many of which make the definition of a sonnet look very loose and informal. In each case, a formal definition acts as a scaffold. It does not define what people who employ the form may express, but it does define, and thus restrict, the means of expression. Above all, the scaffold, by itself, has nothing to say about the quality of the expression. The literary record contains a long list of very bad sonnets: trite, sentimental, poorly expressed, but none the less fitting the formal definition of the sonnet. Conversely, the world contains many magnificent poems that do not fit the definition of a sonnet.