Negative keywords is a critical idea in Google Ads.
In short, using Google Ads you say to Google 'I want to show my ad for users who search for these keywords' - and then you produce that list of keywords.
However, depending on how you provide this list to Google (i.e. what 'match types' you use) you give Google differing degrees of leniency when it relates to matching your intended key words to a user's search.
This can result in your ads showing for searches that may not be actually relevant to what you're selling. Clearly, this isn't something we want to have happen!
For example, you may want to advertise for 'plumber Auckland' because you run a plumbing company that operates in Auckland.
However, Google could assume that a searcher looking for 'plumber jobs in Auckland' is a close enough match and show your ad.
Before you know it, somebody trying to find a job is clicking on your ad (and you wind up paying out). This is not ideal for the performance of your ads, or your bank balance!
This is where negative keywords come in to the equation. You tell Google 'if a user's search incorporates this word (or phrase) then don't show my ad'.
So in that scenario mentioned earlier, you would go about adding 'job' as a negative keyword to prevent your ad showing for people searching for new home building jobs.
This concise video I produced discusses the concept of negative keywords, match types, and why getting this right is so crucial: