Cumulative Layout Shift (CLS) is part of the Core Web Vitals score that Google gives every website. Web Vitals is how Google ranks your site on its technical merits, such as loading speed, ease of usability, and mobile friendliness. https://web.dev/vitals/
Your websites Web Vitals score is calculated with a series of tests and a cumulative score is assigned to your site by Google.
Cumulative Layout Shift, or CLS, measures how much elements on your page move as elements are loaded.
At the time of this writing, CLS accounts for 15% of your sites overall Web Vitals score.