If you have any PopUp fields on this slide, this icon will show while you are chaining. If you have not moved it (them), the button(s) for popping the fields up will appear transparently on top of this icon. Clicking on any such button (wherever it is) will cause the associated field to pop up at this point in the chain.
Clicking onto the field itself while chaining will cause the PopUp field to be hidden at that point in the chain.