Zip Read-Only mode lets you make Zip files read-only (in Opus, that is) to prevent accidental changes to the contents of the Zip file.
The Open Zip files as read-only by default option on the Zip & Other Archives / Zip Files page in Preferences activates this mode by default. If you have this option on and double-click on a Zip file to read its contents in Opus, you will find that you're not able to add items to or delete items from the archive - instead, an error will be displayed.
To make proper use of this mode (unless you just want to have it on all the time, and never make changes to Zip files), you will need to add the Read-Only button to a toolbar or menu. This lets you toggle Read-Only mode on or off when in a Zip file. This command can be found in the Edit category on the Commands tab in the Customize dialog - see the Customize section of this help file for more information.