It's basically screen tearing, apparently because the top and bottom of the display (or left and right in other orientations) refresh at different rates. iFixit suggests its a controller issue.

> Update, 9/28/2021: In response to our inquiry, Apple has told us that the "jelly scroll" issue on the 6th-generation iPad mini is normal behavior for LCD screens.

> Update, 9/30/2021: An iFixit teardown suggests that the iPad mini's more noticeable scrolling issue is a byproduct of how the display controller is mounted.

https://arstechnica.com/gadgets/2021/09/2021-ipad-mini-suffe...

FWIW, my 5th Gen Mini doesn't have this issue.

I'm guessing Apple is using an IPS panel meant for landscape orientation (i.e. the image scans from the top/bottom) in a device mostly used in portrait orientation. This causes rolling shutter distortion when scrolling contents.

They are referring to the screen lagging/tearing while scrolling