Google wants the web experience to be smooth. Smoothness is defined basically by just a few things:
- don’t be slow (to load or show the page)
- don’t be annoying
- let user be at peace, enjoying your content
- keep your page delightful and stable
Addy Osmani nails the Vitals very well in this video:
Jukkasoft 
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[…] Cumulative Layout Shift (CLS) measures jumpiness and jerking of a web page. […]