CLS is a metric used to understand how visually stable a website interface is. CLS is a score that Google uses in their Search Console as an important user-centric, on-site SEO metric. A high CLS score indicates a “spammy” or poorly constructed web page.
As a gauge of user experience and visual interaction, CLS is actually looking at parts of a website moving while a user is trying to interact with them. CLS is not taking into consideration video or moving animations, but rather points of engagement moving or adjusting improperly.
CLS items normally happen with buttons expanding in size or moving, popups interfering with engagements and more.
As mentioned, CLS is a component of Google Search Console under “Core Web Vitals”. Errors here are shown prominently and can be assumed have a significant, negative impact on rankings.