Concerning's Keats's "On First Looking Into Chapman's Homer," I'll add a bit of explanation to the above paraphrase.

The "realms of gold" that the speaker has traveled are metaphorical--he has read a vast amount of classical literature from Greece and Rome.  He is establishing that he is at least somewhat of an expert in classical literature.  That is why, in the sestet (the last six lines) he is so surprised and shocked and impressed at his discovery of how great this particular translation of Homer (Homer's writings) is. 

The first eight lines (the ocatave of this Italian sonnet) establish his expertise in classical literature and how he really knew nothing about it, until he read Chapman's Homer.

The last six lines compare his discovery to great discoveries that other humans have made.    

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.