Dickinson introduces her metaphor in the first two lines ("'Hope' is the thing with feathers/That perches in the soul"), then expands on it throughout the poem by telling us what the bird does (sing), how it reacts to adversity (it is unafraid in the storm), and where it can be found (everywhere, from the "chilliest land" to "...the raging sea"). Now, as far as I know, there are no birds with souls. But the idea that hope is an emotion that we feel inside of us, like courage, fear, or love, is something that many people have said before Dickinson. So she did not invent this concept. Also, please note that this line is considered one of Dickinson's best poems because of its use of language and how it makes you think. And finally, yes, this is a metaphorical line.