Negative keywords are terms you add to a Google Ads campaign to stop your ads showing for searches that contain them, so budget goes to queries that can actually convert.
Every paid search campaign matches more searches than it should. Broad and phrase match are built to reach variations you did not think of, and most of those variations are useful. Some are not. A campaign for "buyers agent" will, left alone, also pay for clicks on "buyers agent jobs", "how to become a buyers agent" and "buyers agent salary". None of those people are hiring. Negative keywords are the filter that removes them.
Google's documentation frames them simply: negative keywords let you exclude search terms so you can focus on the ones that matter. They work in three steps, and the third never ends.
First, look at the search terms report, the list of what people actually typed before your ad showed. This is the campaign's source of truth, and it almost always holds surprises. Second, add the irrelevant terms as negative keywords, "free", "DIY", "jobs", "course", "salary", a competitor's brand. Third, do it again next week. New junk searches surface constantly, so negative keyword work is maintenance, not setup.
Two details decide whether the filter works. Negative keywords have match types, broad, phrase and exact, and each excludes differently: negative phrase match blocks the exact run of words in order, negative exact blocks only that precise query. And the detail that catches people out, negative keywords do not match close variants. Adding "plumber" as a negative does not block "plumbers", a plural, or a misspelling. Regular keywords expand to close variants automatically; negatives do not. You add the variants yourself.
The currency point most guides miss. For its first few years, Performance Max gave advertisers no negative keyword control whatsoever, a real problem for any business with a brand name close to an unrelated search. Google reversed that. You can now apply negative keywords to a Performance Max campaign or account-wide, though they only filter Search and Shopping inventory, not YouTube, Display or Gmail.
What this is worth, concretely. A Brisbane plumber bidding on "blocked drain" who has never added negatives is paying $2 to $4 a click to reach people searching "blocked drain DIY", "drain snake hire" and "blocked drain plumbing course". On a $1,500-a-month ad budget, that leak can quietly swallow a quarter of the spend. Twenty minutes in the search terms report recovers it. Negative keywords do not raise your ceiling, they stop the budget draining out the bottom, which is the cheapest improvement available in PPC and the fastest way to bring down cost-per-click on the searches that count.
Our SEO & AI Visibility service runs this clean-up as part of every paid search engagement.