Negative keywords block your ad from searches containing them — 'jobs', 'free', 'DIY' — so job hunters and freebie seekers never cost you a click. Without them, a meaningful slice of any budget quietly buys worthless traffic.
They behave differently from normal keywords in one trap-laden way: negatives don't match plurals or variants automatically. Blocking 'course' does not block 'courses'; you must list both.
Start with a seed list (ours is free — 53 terms, annotated), then grow it weekly from the search-terms report, which shows the actual searches you paid for. Ten minutes a week here is the highest-ROI habit in paid search.