The speaker begins the poem by setting a dark, ominous scene. "Huge" clouds "brood" above a seashore lined by cliffs. This personification (and more specifically pathetic fallacy) presents the clouds themselves as moody and ominous (to "brood" means to worry or mope). Clouds can't literally brood, of course, and this description seems to reflect the speaker's own state of mind, projected onto the surrounding environment. The poem opens on a foreboding note, vividly conveying the intimidating feeling of staring out at an ocean on a stormy evening.
The second line digs into this feeling, as the speaker describes the night as "dark and mute." The word "mute" suggests that the entire setting is strangely quiet, as if the poem takes place very far away from the standard noises of daily life. Such descriptions draw upon imagery that makes it easier for readers to envision this "dark," silent place.
"Written Near a Port on a Dark Evening" is written in iambic pentameter, meaning that its lines are mostly made up of five iambs (metrical feet consisting of an unstressed syllable followed by a stressed syllable, da-DUM):
Huge va- | pours brood | above | the clif- | ted shore,
Night on | the o- | cean set- | tles dark | and mute
The steady iambic rhythm reflects the rolling clouds and the subtle sound of waves. That said, the meter isn't perfect. The second line starts with a trochee (a foot with a stressed syllable followed by an unstressed syllable) and the first foot of line 1 can also be read as being a spondee (meaning that both syllables are stressed: "Huge va-").
These are minor variations, but they still manage to make the language sound just a little less predictable and, because of this, ever so slightly unsettling.