What is a “chapter abstract”?

An abstract is a short summary of a scholarly work, normally appearing at the beginning of that work.

But a book can contain chapters, and an individual chapter can also have its own abstract—especially when the book is presented on an academic publisher's platform for example when I say:

“Kolchin's chapter abstract”

I mean:

the abstract/synopsis provided for that particular chapter on the academic publisher's website, not the abstract of a research article or thesis.

A journal article normally has:

title and abstract and main text.

An academic book normally has:

book and chapters and each chapter may have its own title and sometimes a chapter-level abstract/summary on the publisher's website.