CLS (Cumulative Layout Shift) is a metric that measures the visual stability of a webpage. It is used to quantify the amount of unexpected layout shifts that occur during the loading of a webpage, which can negatively impact user experience. A low CLS score is generally considered to be good, as it indicates that the layout of the page is stable and elements are not shifting around as the page loads. In terms of SEO, a lower CLS score can help improve the user experience and potentially lead to better search engine rankings.