The poem argues in favor of outsiders and criticizes people who depend on others' attention for approval. It sets up this premise immediately with its emphatic opening statement, in which the speaker introduces themselves with an oxymoron. The speaker refers to themselves as "Nobody," the capitalization of the word making it read as a proper noun (like a person's first name). Not only is the speaker "Nobody," but they're also enthusiastic to say so, as shown by the exclamation mark caesura in the middle of line 1.
In essence, this opening is a kind of parody. Overall, the poem questions those who are overly reliant on external approval—people whose lives are governed by garnering attention and being liked. This type of social behavior—at least from this poem's perspective—is a kind of performance, and the first line seems to deliberately mimic such behavior. It's almost like the opening remark of one businessperson meeting another for the first time, making sure that the other is under no illusion as to the first's identity. It echoes the language that two people might use when shaking hands and gives the impression that they're trying to impress each other. But while the line has the tone and insistence of a confident meeting, it is actually a deliberate expression of anonymity. That's why it's an oxymoron—you can't be "nobody" in the literal sense if you are a living, talking human being saying the word "nobody" out loud (or writing it on the page).
With that in mind, then, "nobody" already means something different from what it might mean in a sentence like "nobody was there." Whereas in that example, "nobody" denotes an absence (there were no people around), "Nobody" here does precisely the opposite—it announces someone's presence.
Following this proclamation, the speaker asks the reader—or an off-page addressee—who they are. In a way, this is a rhetorical question—the poem provides no answer in textual form, and whoever is on the receiving ends of the question can't answer. But of course, the rest of the poem undercuts this brilliantly by providing its own answers. This begins in the second line, with the speaker asking hesitantly whether the reader is a "nobody" too. The two caesurae in this line—the characteristic Dickinson dashes—create a sense of hesitation. This is important, because the poem overall is implying that most people are not willingly "Nobodies," but rather prefer to be "Somebodies." The speaker is so amazed to find another "Nobody" that the basic question ("Are you nobody too?") doesn't come easily—it's almost like the speaker is checking that this other "Nobody" is real.
The word "too," though small, is crucial. It speaks to a process of recognition, as the speaker realizes that he or she isn't the only "Nobody" in the world. Thematically speaking, these lines establish a sense of solidarity between two people, showing the reader how connection can occur without seeking the approval of others. This is the poem's central paradox: a community of people based on anonymity, who, by virtue of their shyness (or just personal preference) are unlikely to ever actually meet face-to-face. Dickinson's inability to find literary fame during her lifetime is also relevant. People who don't seek fame and attention, the poem seems to suggest already , still make an important contribution to humanity and are capable of living happy lives.