What is a negative keyword?
A negative keyword prevents your ad from appearing for searches containing that word or phrase.
They exist to stop you paying for the wrong intent. Someone searching 'free', 'jobs', 'salary', 'DIY' or 'how to' is not about to hire you, and every one of those clicks costs the same as a real prospect's.
Two categories are worth building. A standing list covers universally wasteful terms. A business-specific list covers adjacent services you don't offer — the expensive mistake, because those searches look perfectly on-industry while being completely unqualified.
On a small budget this is the highest-return work available. Blocking a term that was quietly consuming a fifth of the budget improves everything downstream without touching bids, ads or targeting.