What is a "Graphical Table of Contents" Abstract?
Some publishers (ACS I'm lookin at you) have what they call GToC, which is like a ToC with small images. You might have seen such in some magazines, here's an example.

Accompanying the image is usually a short text that is sort of like a teaser for the article, in a way the coolest tidbit about what you have written. I believe that's what they are asking for here.