A frigate is a ship, and in her poem, Emily Dickinson is comparing it to a book because of a book's capacity to transport readers to places far and wide in their imaginations. Dickinson says,
"There is no Frigate like a Book
To take us Lands away."
Dickinson takes the metaphor a step further in the next two lines of the poem when she compares a page of poetry to a "Courser," which is a swift horse. Like a frigate, a courser is a mode of transportation, and Dickinson reiterates her contention that a book, and specifically a book of poetry, has the potential to transport readers to places they might never see except in their imaginations, as stimulated by what they read.
In the last four lines of her poem, Dickinson stresses that the magic of books is available to everyone, rich and poor alike. The journey of the mind that books provide is one that even the poorest may take, without having to worry about money, the "oppress of Toll." Even though the passenger taken on this journey is of great value, the "Human soul," a book, which is now compared to a "Chariot," is a frugal or inexpensive mode of transportation indeed.