Cumulative Layout Shift (CLS) is a metric used to measure the visual stability of a webpage during the loading process. It refers to the amount of unexpected layout shifts that occur as a page loads, causing content to move around on the screen. These layout shifts can be frustrating for users, especially when they cause them to click on the wrong button or link. The CLS score is calculated by multiplying the impact fraction of each layout shift by its distance fraction and adding up the results over the entire page load. A low CLS score indicates a more stable and user-friendly webpage, while a high score suggests that the page is more likely to cause frustration or errors for users.