What does this thin purple border line indicating?

Quote from MisterK

Layout with Preview off. What does this thin purple line around the page indicate? It goes away when I turn Preview on.

The thin purple line indicates the bleed,
an extra margin of a few millimeters that is added to the artwork file beyond the final dimensions of the printed product,
extending colors and images up to that limit to prevent unwanted white edges due to the slightest inaccuracies in mechanical cutting.

Bleed can be set in the document settings, either when creating a document or adjusted later. When preparing a document for print, it’s generally a good idea to check with the print shop in advance to confirm the required bleed size.