The icon indicates that your iPad is synchronising with "something"...
Yours is one of many reports seen in recent weeks. At present, pending confirmation by Apple, this appearance of the icon is believed to have some relationship to either (a) post-update synchronisation tasks performed by your iPad after updating to iPadOS 18.x - or (b) the icon appears while a file download is being performed as a background task.

Apple has not yet provided a definitive explanation as to the meaning of this status icon. Appearance of the icon is benign; it will disappear as soon as the related background task has completed successfully.
Apple's documentation is often slow to catch-up with new features. Here is another status icon - that appears briefly when connecting some USB-C hubs - that Apple have yet to add to their documentation:
