From Cloudflare: "Cumulative Layout Shift [CLS] measures visual stability [of a web page when it is loading in a browser]. CLS measures how much a webpage 'jumps around' as it loads. A good CLS score is 0.1 seconds or less. CLS is a somewhat complex measurement." The Web Dev CLS article below has a more thorough breakdown of how CLS is measured, but the general idea is to aim for all webpage elements to load as close to simultaneously as possible, and in their correct positions.
