Plain answer
Negative keywords are the searches you tell Google never to show your ads for. Bid on "plumber" and add "jobs" as a negative, and job-hunters stop costing you clicks. They're the main defense against paying for traffic that was never going to buy anything.
How negative keywords actually work
Every keyword you bid on is an invitation; match types decide how loosely Google interprets it. Negative keywords are the veto. Add a term as a negative and searches containing it are excluded, whatever your match types would otherwise allow.
The usual suspects for a business account: "free," "jobs," "salary," "course," "DIY," "how to" — searchers looking for employment, education, or a way to avoid paying you. None of them are bad people. They're just not customers, and without negatives you're funding their research.
How do you find the negatives you need?
Not by brainstorming — by reading. The search terms report lists the real searches that triggered your ads and what each cost. Scan it weekly, flag everything you'd never want to pay for again, add those as negatives. The first read of a neglected account is usually eye-opening; that's the exercise where wasted spend stops being an abstraction and becomes line items.
Field note from TomasI recently audited a B2B fintech account running on autopilot: the keywords were right, but loose match modifiers let consumer traffic pour in and nobody had excluded it. Roughly 80% of the budget was going to people who could never buy. The fix was sitting in the search-terms report the whole time — and afterwards, performance improved so much the company had to hire more salespeople to handle the demand.
Can you overdo negative keywords?
Yes. Blocking aggressively on guesses can strangle a campaign's reach, and negatives don't expand to close variants the way biddable keywords do — you often need the plural and the misspelling too. The discipline is to block what the data shows is waste, not what you imagine might be.
What this means for your ads
Highest-return maintenance in search advertising: ten minutes a week, applied straight to the leak. If nobody has looked at your search terms report in months — you, an agency, anyone — assume part of your budget is quietly buying searches you'd veto on sight.
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