What does this poem by Edgar Allan Poe mean?

It means that he laments a hateful word spoken to a loved one and of the regret that the love of that person is lost to him. He ironically asks that those who "p him by" (people who don't know who he is, what he has done or what he has lost) not to be concerned about the consequences of his unthinkingly hurtful remark. It's more or less a warning.