The poem's title, "Recuerdo," means "memory" or "I remember" in Spanish. Right away, then, readers might get the sense that this is going to be a poem about an important moment in the speaker's life, one that they want to hold onto.
The poem was supposedly based on a night Edna St. Vincent Millay spent with Salomón de la Selva, a Nicaraguan poet and friend, during which they rode back and forth on New York's Staten Island ferry in New York. Readers don't need to know this context to understand these opening lines, however, which describe the speaker and a companion happily riding a "ferry" to and from its port "all night."
This might not seem like the most enjoyable way to spend an evening, especially given that the ferry "smelled like a stable." Indeed, the speaker makes it clear that this is no fancy cruise liner; the ferry is "bare and bright," free from cozy decoration or soft lighting. The bold alliteration of "bare and bright" and the hissing sibilance of "smelled" and "stable" call readers' attention to just how physically unpleasant the onboard experience must be.
Most people taking the ferry would, quite reasonably, have just been trying to get to the other side. For the speaker and their friend, however, the night is about the journey more than the destination—and the journey is just a way for them to spend more time together. It sounds like the kind of thing broke students or artists might do to pass the time.
The speaker and their compansion are thus perfectly content, "very tired" but also "very merry." The anaphora of "we were very" adds a kind of sing-song rhythm to the moment, establishing an atmosphere of carefree joy. The same is true of the musical internal rhyme between "very" and "merry." Line 2 then once again begins with "we," another example of anaphora.
The abundance of "we"s in the poem is no accident: this is a poem about the beauty and power of companionship, so it makes sense that the speaker repeatedly points out that they're part of a "we." That "we" foregrounds the reason why the speaker remembers that night fondly: who it was spent with. Even the poem's form echoes the importance of companionship: "Recuerdo" consists of rhyming couplets—rhymed pairs—from start to finish.