The poem begins with an anonymous speaker describing a certain kind of "Light" that only appears in early spring. More precisely, this light shows up in the first days of March—when the month has "scarely" begun.
Spring is the time when animals emerge from hibernation, the leaves return to the trees, and sprouts begin to shoot from the thawing ground. Coming on the heels of cold, dark winter, the season typically symbolizes rebirth and fresh starts. Light, meanwhile, is typically linked with truth, knowledge, understanding, and holiness.
Right away, then, the reader might sense that there's something uniquely invigorating or divine about this light. At the same time, the speaker makes it clear that this light won't stick around; it's present only in the very early days of the season. This suggests that whatever it is the light may represent to the speaker—insight, revelation, or even a connection to god—it can't last for very long.
The sounds of this opening stanza help to create a quiet and meditative as the speaker reflects on this special, ephemeral light. Listen, for example, to the gentle sibilance that fills these lines:
A Light exists in Spring
Not present on the Year
At any other period—
When March is scarcely here
This opening stanza also establishes the poem's form: its 20 lines are broken into five quatrains, or four-line stanzas, that follow an ABCB rhyme scheme (lines 2 and 4 rhyme with each other, while lines 1 and 3 do not). The poem is also written primarily in iambic trimeter: lines of three iambs (poetic feet made up of an unstressed syllable followed by a stressed syllable: da-DUM). Every third line is written in tetrameter, however, meaning it contains an extra iamb. Here's the first stanza scanned:
A Light | exists | in Spring
Not pres- | ent on | the Year
At an- | y oth- | er per- | iod—
When March | is scarce- | ly here
This quatrain thus looks a lot (though not exactly) like the ballad stanzas that Dickinson so often turns to in her work. Its rhythms sound easy and familiar.