What is Layout shift?

Also called CLS, Cumulative Layout Shift.

Layout shift is content moving on the page while it loads — a button sliding down as an image finishes arriving above it. It is measured because it is the thing that makes people tap what they did not mean to tap.

In more detail

The usual causes are images without width and height attributes, web fonts swapping in at a different size, and banners or embeds injected after the page has rendered. The fix in every case is reserving the space in advance rather than letting the element claim it on arrival.

Why it matters for your site

Beyond the ranking effect, this is a direct conversion problem: a checkout button that moves at the moment somebody reaches for it costs you that sale. Setting dimensions on images usually fixes most of it in one change.