It's mysterious, unpredictable, and a little scary. It can come out of nowhere and drop right on your head without much of a warning. And then, like a kitty, it moves on leaving us kind of in awe as we wonder where it's going next. Wrapped up in the fog is its uncertainty, and maybe even questions about similar occurrences that happen in life that seem to come out of nowhere and then disappear. Seeing that the fog is everywhere in the poem, we get the sense that it functions poetically in the same way it does in nature. It envelops nature just like it does the poem (and our imaginations).
Title: It's first in the title. So in a way the fog hovers over the poem in the same way it does in nature.
Line 1: The fog "comes" which puts us immediately in the perspective of watching its approach. We're not sure where it will end up, so we're already feeling a little anxious and curious about its movement.
Line 2: It comes on "little cat feet" which is a metaphor that suggests it's quiet and a little sneaky in the way it creeps up on us. It's not yet enveloping the city, but it's on its way.
Lines 3-4: Now that it's here, we feel that some time has passed between its approach and arrival. It's "sitting" and "looking" over the city and harbor which adds to that looming anxiety as to what it will do next. Maybe it will fall on our heads, maybe not.
Lines 5-6: Those "silent haunches" add to our sense of the fog being a little dangerous because it's so unpredictable. It can pounce on us with those "haunches" or slink away. By the last line it "moves on," and we breathe a sigh of relief.