A negative keyword works as a filter.
You can use it to remove themes that don’t fit what you’re doing to ensure your ads will not be shown for those themes. It’s when you put the minus sign ahead of the word. Use this to filter customers and make sure that we get the targeted customers that we are looking for and we're also going to need to learn when to use which negative match type.
Match types:
Broad negative keyword
Phrase negative keyword
Exact negative keyword
For specific words the broad negative is always the way to go.
Find those specific words that you don't want your ads to trigger for (e.g. discounted, sales, free) and then you're gonna want to make a negative broader list for those words.
Find negative keywords
You can look into Google's related search terms (found at the bottom of the page when you try to search for something) and see what keywords you want to exclude.
2-1 rule — for every keyword that you find and for every keyword that you put into an ad group you are going to want to have at least two more negative keywords.
Our keywords are only as effective as our negative keywords matching up to them. If you want to be really specific and you want to really optimize your ad group you need to be proficient with your negative keyword.