Thanks, Auctor! Really appreciate it.

The short answer: Invisible keywords are untapped keywords. They are topics people search for on Google but no one else has directly answered them yet.

I'll probably write an entire article about them, but here is an article about the blogging course that taught me how to find them:

https://medium.com/the-bald-writer/i-spent-299-on-a-niche-blogging-program-heres-what-i-learned-2debfebac0e