A word or phrase that people commonly search for when looking for things on the net. When you write keyword-rich content for your site or in your site’s blog, you’re niching down your site which helps google to understand what your site is about. Thus, allowing Google to offer your content to potential customers who are searching for content related to your site’s content.
Also, using relevant keywords throughout your site will help you with ranking your site for SERP.
In paid search advertising, this word or phrase is used to match a search query to an ad. For search engine optimization, this word or phrase is used to create content that will be relevant to users when found in a search query.