Right from the beginning, the poem creates an atmosphere of anticipation. The first two lines of the first section are entirely descriptive, establishing the poem's distinct and mysterious sense of place. Of course, this isn't hollow descriptive language—the purpose of the description will become clear later in the poem. For now, though, all readers get is the image of a shoreline at night.
The nouns here are purposefully vague. The reader is presented with "sea" and "land," but no indication of which sea or which land. This creates a sense of mystery and risk—wherever this landscape is, it is an unfamiliar place. At the same time, these words also suggest that the specifics of the location are not all that important to the speaker. That is, the ominous landscape is nothing more than another obstacle in the speaker's journey. It's not important which sea or land the speaker is traveling through—it's where the speaker is traveling toward that matters most.
In addition to the title, the first two lines make it clear that the poem takes place at night. The land is visible only as a kind of black mass, emphasized by the way that /l/ sounds are drawn out by alliteration and consonance in "long black land." The adjectives, then, are an important part of creating the poem's specific atmosphere—and the low visibility suggests uncertainty and potential risk. It also suggests secrecy—that something about this meeting is clandestine, because it has to happen under the cover of darkness.
Line 2 picks up on the /l/ in line 1, with nearly half of the words in making use of the same sound "yellow half-moon large and low"). Here, the sound works to create a sense of the imposing sight of the moon, hanging "large and low" in the sky. The line also introduces another key technique that the poem uses to develop its atmosphere and heightened suspense. The line begins with "and," introducing the poem's polysyndeton. This repetition of "and" suggests the seemingly endless nature of this journey—it is one step, followed by another, and another, and another.