At first, "Fog" seems like a fairly simple poem. There's nothing especially unusual about the title or the first line: "The fog comes."
But the poem's seemingly straightforward, bare-bones style quickly shifts and becomes more interesting. Line 2 introduces a striking metaphor, comparing the fog to a cat. What kind of fog, readers might ask, appears "on little cat feet"? In what ways can fog be like a cat? Why compare fog (an inanimate, often inconvenient weather event) to a cat (an often lively and beloved animal) in the first place?
These first two lines don't necessarily answer those questions, but they do establish the fog/cat parallel that will continue throughout the poem, becoming an extended metaphor.
These lines also contain lots of alliteration and consonance: notice the repeated consonant sounds in "fog" and "feet," "comes" and "cat," and "little cat feet." These sounds announce the opening of the poem, the appearance of the fog, and the introduction of the fog/cat metaphor. The tripping/t/ sounds, in particular, mirror the pitter-patter that "little cat feet" might make so that the poem itself seems to arrive in the same delicate way as the fog.
This first stanza (a couplet) also establishes some characteristics of the poem as a whole. "Fog" is a free verse poem, meaning that it follows no set rhyme scheme or meter. Several of its lines (including line 1) are enjambed, meaning that their clauses continue across the line break (the end of the line doesn't coincide with the end of a clause or sentence). These effects give the poem a plainspoken and flexible quality; they're also hallmarks of the style that made Carl Sandburg an acclaimed, popular poet.