Negative keywords filter out unwanted impressions to help you reach the most relevant audience, just by adding them to a ad groups or campaign means that the ads won't show for searches containing those term.
"The process to add negative keywords to a ad group is just like adding other keyword —with just a difference is that you put negative sign (-) before a keyword. "Adding to ad group level means the term will only affect ads in the ad group. And, at campaign level, all ads in all the ad groups in that campaign affect." You can easily identify potential negative keywords for your account! by just using Keyword Tool and Search Query Performance report for ad groups. Using these tools to add negative keywords on regular basis is recommended to ensure you don't accrue unwanted clicks and, with a limited budget and a maximum CPC of $1, negative keywords will ensure you effectively utilize your budget," explains Google.
Tips to Optimizing AdWords Campaign: Help Center