This poem with only two stanza is very concise and simple in the literal sense of the word. We can characterize the speaker as a person who wants to be "nobody," and she says something ironic about "somebody," in the second stanza.
In the 1st stanza, she makes herself clear by telling us that she is "nobody." When she asks, "Who are you? Are you Nobody too?" she wants to make sure about our identity. She is inviting us to be her company so she says, "Then there's a pair of us!" However, she doesn't want "somebody" to know that she and her new friend (the readers) are "nobody," so she warns us not to tell.
In the 2nd stanza, she thinks that it is horrible to be "somebody," because "somebody" is like a frog who croaks all the time to prove its existence and who is admired only by other small animals in the damp bog. In this case the bog symbolizes the value of our society. If someone does things according to the value of the society, he will be honored by others.
The dashes indicate both hesitation and secrecy. Since she doesn't want "somebody" to know that she is "nobody," the dashes before "Then there's a pair of us," help us to imagine the scene when she questions hesitatingly. I think when she says, "they'd banish us," we may or may not know immediately what she really means. Nevertheless, by the dash and "You know!" she automatically includes us and lets us think that we really are alike. The latter dashes also help us to imagine that she is talking to her friend secretly about her idea of becoming "somebody."
We can consider "somebody," as a person who wants to be famous, and "nobody," as a person who doesn't care about the worldly affairs. Once a person becomes "somebody", he/she will be respected and valued. On the other hand, there will also be some sacrifices to make; one of them is the risk to lose one's true self, because becoming "somebody" needs other people's approvals. Maybe he has to pretend that he is competent, capable and productive, otherwise he would no longer attract people's attention and might be trampled upon by other competitors.
After reading this poem several times, I start to think how strange it is to assert oneself to be "Nobody," if she really wants to be nobody. By capitalizing "Nobody" and "Somebody," the speaker puts a lot of emphasis on the meanings of them. I hear many people say that they "only" want peaceful lives instead of becoming well-known. However, peaceful lives are often more difficult to have because our lives which are also normal or ordinary, are not peaceful at all. Do you understand? I don't know how to explain it but I think the speaker wants to say that actually "Nobody" is "Somebody." Honestly speaking, most of us want to be "somebody," because we want other people's attention to prove our existence. In another point of view, wanting to be a "Nobody" is more heroic to do.