What is induction-induction?

Question

What is induction-induction?

The resources I found are:

• the HoTT book, at the end of chapter 5.7.
• nLab's article
• a paper called Inductive-inductive definitions
• this blog post also mentions inductive-inductive types

The first two references are too brief for me, and the latter two are too technical. Can anyone explain it in layman's term? It would be better if there's Agda code.