User blog comment:MisterWoodhouse/The first migrations to the Unified Community Platform/@comment-45763466-20200808003117/@comment-37336800-20200808004300

Basically it means you can't edit/create pages until it goes out of read only mode. You can still read them though. So for a couple hours (more or less depending on the wiki's size) you will be unable to edit while the wiki gets upgraded.