"So We'll Go No More a Roving" starts with a firm decision: not to go "roving / So late into the night" anymore. In the most basic sense, "roving" means wandering, but here it implies partying, pleasure-seeking, and carousing: all the fun of nightlife.
The speaker, who speaks for a collective "we," doesn't yet explain why they've made this resolution. But perhaps readers can get some context from the poem's allusion to an earlier Scottish ballad. In that ballad, the words "And we'll gang nae mair a roving / Sae late into the night" (the Scots version of this poem's first words) form a refrain between verses that tell a story of sexual mischief and good times. Maybe this speaker's "roving" had a similar flavor.
But Byron's poem doesn't explain any background: it just jumps right into the middle of things with the word "So." This abrupt opening helps grab the reader's attention and pique their interest about what could have prompted this sudden decision.
The "So" in line 2 is interesting as well. The speaker isn't claiming that they'll stop roving altogether, or roving "at night," or even roving "late into the night." They're just claiming that they'll stop roving "So late into the night." They might still have wild nights out, but those nights will end a little sooner. That's a pretty limited pledge! This language sets up the subtle conflict that runs throughout the poem: this speaker sounds like they intend to embark on a new, more mature, slower-paced phase of life, but part of them is clearly reluctant to do so.
The sounds as well as the words here introduce the poem's jaunty-yet-wistful tone. Take a look at how the meter works in these first two lines:
So, we'll go | no more | a roving
So late | into | the night,
This energetic pattern of accentual trimeter, in which lines always use three stresses but switch up their metrical feet (i.e., those stresses don't always fall in the same place in a line), gives the poem a bouncy, playful, musical sound. Meanwhile, strong assonance on the long /o/ sound in "So, we'll go no more a roving" adds a touch of mournfulness, as if the speaker is sighing "Oh!" In the tension between its lively rhythm and its poignant music, the poem establishes a mood of tongue-in-cheek nostalgia right from the start.