A SERP tool is a software program that helps you improve your website's search engine ranking. It does this by identifying the keywords and phrases that your site is currently ranking for, as well as the ones you're not ranking for. It then gives you suggestions on how to improve your site's ranking for those keywords and phrases. One of the most popular SERP tools is called SEMrush. It has a free version and a paid version. The paid version gives you more features, such as being able to see the estimated traffic a particular keyword brings to a website.