What is the poem there is no frigate like a book about?

THERE is no frigate like a bookTo take us lands away,Nor any coursers like a pageOf prancing poetry.This traverse may the poorest takeWithout oppress of toll;How frugal is the chariotThat bears a human soul!

The first stanza refers to that sensation of being somewhere else, while reading a book; of losing yourself in its pages. The second stanza is just saying that everyone who can read, can take this 'voyage', and therefore, no matter your financial position, you can go on a journey to wherever you like. Emily Dickinson is comparing books to boats by saying that they are both vessels in which can take you somewhere else, books in a mental journey, and boats on a physical journey.