Personification is a literary device in which human characteristics are attributed to something non-human, e.g. an animal, object, or idea.
It is used in the given line from Sara Teasdale's poem <em>Barter</em>. The white hour of peace is described as <em>singing</em>. An hour can't sing in real life. It's something that people do. This is why the hour can be described as personified.