The poem opens with the speaker's surprising confession:
I dreaded that first Robin, so
In other words, the speaker was not looking forward to the coming of spring—a season whose beginning is traditionally marked by the first sighting of a robin.
Already, then, the poem challenges the reader with a question: what could make the speaker feel this way? It's hard to guess why anyone would be afraid of a cute little robin or of the spring more generally.
The rest of this first quatrain doesn't provide much insight. The speaker claims that she's faced up to her fears and "mastered" the robin she feared so. But to say the robin is "mastered," readers quickly gather, might be overstating things a bit. The speaker has only grown "some accustomed" (a little bit used) to the robin's presence—and he still "hurts a little."
Though the speaker is being mysterious about why this robin might cause her pain, readers might already have a guess: this spring is not one she wanted to face. Over the course of this poem, she'll give a halting, pained account of what it's like to encounter a new season whose bright liveliness clashes with her own darker feelings.
Dickinson writes this poem in her characteristic common meter. That means that each of this poem's quatrains (or four-line stanzas) is written in alternating lines of iambic tetrameter and iambic trimeter—in other words, lines of four iambs (metrical feet with a da-DUM rhythm) alternating with lines of three iambs. Here's how that sounds in the first two lines:
I dread- | ed that | first Rob- | in, so,
But He | is mast- | ered, now,
As is often the case in Dickinson's work, this form—also known as ballad meter, since it's often used in folk songs—will become a deceptively simple vessel for deep feeling.