When you run it, it should load up your current custom auto-complete dictionary (that's the list of terms that you've used in any auto-complete enabled application which aren't in the base dictionary).

You can then select terms from the list and delete them (e.g. passwords that have been automatically added at some point, or misspelled words), or add new terms to the dictionary (this shouldn't really be necessary though, as any terms you use in applications will get automatically added).

Finally, click the "Save changes" button to write out the new dictionary and force the OS to reload it.