what does the * mean when I see it in the word dictionary?

A lot of the words in the dictionary have an * in them, typically (always?) at the front.

What does that mean?

That this word is tacked onto the end of some other word?

Generally, it could be a sign that inform you this word is untested or just a proper speculation based on an ancient form which is now hard to determine the exact form.

Annon allen for Kinnuch's explanation

Or more detailed from the about page:

Unverified or debatable glosses

You'll sometimes encounter the symbol *, usually together with a warning. These exist to inform you that the gloss originate from a source which might be outdated or questionable. This is unfortunately fairly common because linguistic material on Tolkien's languages are only sporadically made available to the community; initiatives have the time to arise and gradually wither between publications. Hiswelókë, which haven't been updated for years, is nonetheless still excellent, and one of the prominent sources to date on the Sindarin language.

Would it be a mistake to trust glosses from an outdated source? Probably not. I recommend you to try to find another source which corroborates the proposed translation.

This also occures for some perfectly fine words, just because Parf Edhellen has no perfect way to filter them out.