The negative keyword list is a very powerful tool. It prevents your ad from showing, when a search query contains any keyword phrase on the list.
For example, here are just a few words from my Negative Keyword List for Products: advice, compared, refund, returns, rip off, scam, reviews, about, cheap, sample, free, instructions, manual, service, courses, homemade, serial number, statistics, tips, build, clubs, compare, consignment, defined, etc. There's over 400 words on it!
If I'm selling a product and paying by the click, I certainly do not want my ad clicked if someone is looking for free samples or the product manual. The same goes for reviews and advice, since I'm selling to people who know what they want. But if you do have a review site, then you will want those clicks.
Instead of having to create your own negative keyword lists, just do a search and you'll find plenty of free examples in many product categories. Go ahead and modify them however you want, then upload them to your WADS account, to prevent your ad from showing when that search term is used.
A champion is defined not by their wins but by how they can recover when they fall.