Hhmm....I'm only familiar with simple HTML and CSS. So I don't clearly understand what this extension does. But from what I can tell, this is the directory (folder) where you want your slices to live. I guess this extension will save the slices (the pieces that will create your webpage) in a single folder, and you need to tell Inkscape which folder to put them in.
I don't understand if this extension actually writes the HTML and CSS, or if you have to write it. Hhmm, time to look at the manual, and
it had some information about this extension.
Oh, good news! If you read English, here's the link:
http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Extensions-Web.html#Extensions-Web-SlicerThe manual is also available in Spanish, French and Italian. But since I can't read those languages, I don't know if this chapter is available in those versions.
I hope that will help you