The poem starts by establishing the image that will develop into the its extended metaphor: “A noiseless patient spider,” which the speaker notices standing all alone on a small ledge (“a little promontory”).
While letting readers know that the speaker is looking at a spider, the complicated syntax (basically, the arrangement of words) here also immediately suggests that something else is at work. A more syntactically straightforward sentence might begin: “I mark’d where, on a little promontory, a spider stood isolated.”
Instead, the speaker's “I” follows after the "spider," as though the “I” is the "spider." That is, it feels for a beat as though the opening line is describing the speaker; think of how someone might say, "A lover of spaghetti, Mary aways went to Italian restaurants"—that first phrase describes "Mary."
The two—the spider and the speaker—are further linked by the assonance of the long /i/ sounds in both words. The adjective “noiseless” to describe the spider is also interesting; rather than an adjective like “silent” or “quiet,” the negative form of “noiseless” gives readers a sense of what the spider is not, or of what the spider is refraining from doing. This creates a sensation of absence (the absence of noise, specifically), that reflects spider's isolation.
The adjective “patient,” meanwhile, personifies the spider, giving it a sense of dignity and consciousness. That the spider is placed on a “promontory” is also suggestive. A promontory is a place of high ground (a kind of lookout) that people usually think of in human terms (a high rock, a headland, a bluff). This description of where the spider “stands” thus further personifies it and ennobles it at the same time. All of this, in turn, will serve to make the poem's extended metaphor, in which the spider and speaker's soul are compared, clearer.
In the third line, the speaker repeats the word “mark’d” to tell readers more of what the speaker has noticed. This repetition, which functions anaphorically (even if the “mark’d in the previous line is preceded by “I”), reminds the reader that the speaker is right there in the scene, watching the spider. It also gives the poem an energy of repeated outward momentum that predicts the outward-reaching action of the spider.
Finally, the spider’s surroundings are described as both “vacant” and “vast.” The alliteration here adds emphasis to the sense of emptiness, even as the shift from the long /ay/ sound at the beginning of “vacant” to the short /ah/ sounds in its second syllable and in “vast” imply a sense of movement and change.