Just think: if you've never actually been out of your home country or even your hometown, you've still traveled far and wide – as long as you're a reader. Think of the places you've "seen" and the people you've "met" while reading. From encounters with fauns and talking animals in Narnia to the bullfight arena in Hemingway's Spain, there's no limit to the exciting explorations you can undertake by just cracking open a good book. "There is no Frigate like a Book" reminds us of the sense of unlimited possibility that reading gives us. You may not be physically leaving your cozy armchair when you read, but you're covering miles and miles of new territory in your imagination.
The speaker values the imagination more than the real world.
The speaker believes that one can travel farther and change more through his or her mind than one can travel through actual transportation.