Hello,
When you add a negative keyword with brackets, it added as exact match negative keyword. (e.g. -[women's hat]). You could add the exact match negative keyword to prevent your ad from showing to people who searched for that exact keyword.
With broad match, your ad may show if a search term contains your keyword terms in any order, and possibly along with other terms. Your ads can also show for close variations of your keywords. With exact match, your ads can appear only when someone searches for your exact keyword, without any other terms in the search.
Thanks,
Umesh, AdWords API Team.