This dialog is available from the File menu of the TOC & Index Editor. It allows you load a TOC or Index by parsing Help 3.x tags from your Help 3 HTML files.
In MS Help 3.x, TOC, visible keywords (Index) and F1 keywords are defined using meta tags stored in the <head> section of your topic HTML files. To edit this meta data FAR allows you to import them into the FAR TOC & Index editor. Once you are finished editing you can Export them back out to topic meta again.
In MS help 2 we also had Associated Link Keywords ("A" Keywords). This type of Index is not supported by MS Help 3.
See also
MS Help 3 Primer