This dialog is available from the File menu of the TOC & Index Editor. It allows you save TOC or Index information (what ever is currently loaded) as Help 3.x meta tags in your 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