dream within a dream

I think Poe is referring to the idea of disillusionment. It seems Poe is saying that the hope we think we have for our futures and for things that could come to pass was maybe never there to begin with. What we thought we had was only a false hope and a sense of knowing that which we couldn't possibly know. He is telling us that we are grasping, but at what? What we thought was there may have only been a dream.
This idea can apply to any dream, but especially the American dream. Everyone has an idea of what they want and feels "hope" to get to that plateau. Once there they discover they only thought that's where their hope lay. The hope they had did not produce the end they desired.