First two lines: I've travelled around and seen many rich kingdoms
Next two: I've been around many of the islands of Greece (held by poets who are faithful to Apollo).
Next two: I'd heard alot about a place that Homer ruled as his own.
Next two: But I never understood about this until I read Chapman's translation.
But when I did, I felt like some astronomer who has discovered a new planet or like Cortez felt when he first discovered the Pacific (it was Balboa, not Cortez...)
Overall, he is saying that reading Chapman's translation of Homer opened up whole new worlds to his mind.