Syntax refers to sentence structure and, specifically, the order in which words come in a sentence. Therefore, we might talk about regular syntax, what is a typical word order in a given language, or irregular syntax, what is an atypical word order. e.e. cummings is known for his irregular syntax because it is such a consistent feature of his poems. You may notice, for example, that this entire poem is actually only one sentence; it is quite complex, with semicolons and colons, independent and dependent clauses, but it is one sentence nonetheless. Typical sentences are infrequently this long or complex, and that alone makes its syntax unusual. However, in the third stanza, the narrator says,

or if your wish be to close me,i and
my life will shut very beautifully,suddenly,
as when the heart of this flower imagines
the snow carefully everywhere descending . . .

A more typical syntax would be to say that the snow is descending carefully everywhere rather than carefully everywhere descending because carefully is an adverb that describes the verb descending; in cummings's text, however, we get the modifier before we get the word it modifes. By inverting typical word order patterns, cummings draws more attention to the language he's using and adds a freshness to his work.