What is the subject of "Hope is the thing with feathers" by Emily Dickinson?

The simple answer is that the subject of this poem is a description of hope. By comparing hope to a little bird, Dickinson describes what it is without actually saying what she is doing.

The first two stanzas read:

“‘Hope’ is the thing with feathers –

That perches in the soul –

And sings the tune without the words –

And never stops at all –

“And sweetest - in the Gale - is heard -

And sore must be the storm -

That could abash the little Bird

That kept so many warm – ”

(Poetry Foundation)

When she says, “sweetest – in the Gale – is heard”, she emphasizes that hope prevails even in the midst of storms. In fact, this is often when people notice hope the most. It is when things are hard that one realizes he has to hope.

“And sore must be the storm – That could abash the little Bird That kept so many warm”. Here, Dickinson points out that a storm must be exceptionally harsh to crush the flame of hope that exists during trials. Because of hope’s unconquerable nature—the necessity of people to have hope in something—it really takes a lot to extinguish such a powerful emotion.

Here is the third stanza:

“I’ve heard it in the chillest land -

And on the strangest Sea -

Yet - never - in Extremity,

It asked a crumb - of me.”

(Poetry Foundation)

In this verse, Dickinson stresses the unique places hope’s sound is heard. It appears in unusual locations—places where it is needed most, and where one might not expect to find it. Her last two lines imply that while hope provides a level of comfort during difficulty, it is not something that taxes you or demands something from you (see Brooklyn College’s analysis).