What do you think this quote could mean?

It could mean that there are other members of the Ghidorah species and that King Ghidorah left his home planet in order to rule over another.

@SasquaDash: I agree! Do you think it’s meant to set up the arrival of Keizer Ghidorah for a future film? It came from the same book that included a moment where Dr. Serizawa remembers his son Ren, who later shows up in GVK. 

I believe that this quote could be an introduction to Ghidorah's own movie or a prequel to an existing movie. 

Thanks for the help in interpretation. I could not understand for a long time what this quote could mean too... I remembered this site https://freebooksummary.com/category/the-crucible-quotes, which often helped me understand the meaning of this or that phrase from any work. For example, there I found many quotes from The Crucible, and on the site, I also read their meaning. Try to search there and about Godzilla. Good luck!

I also think it's an introduction to Ghidorah's movie.