Cumulative Layout Shift (CLS) measures the visual stability of a webpage. It quantifies how much the content shifts during loading, which can lead to a poor user experience. A high CLS score indicates that elements on the page move unexpectedly, causing users to click on the wrong items or lose their place. CLS is part of the Core Web Vitals, a set of metrics that assess the performance of a website. To improve CLS, web developers can ensure that images, videos, and ads have defined sizes, preventing layout shifts as the page loads.