To undo the process of reading.
"The software glitch caused the document to become unread, reverting all the changes I had made during editing."
In plain English: To unread something means to take back information you previously read so it doesn't show up as seen again.
"I decided to leave my book unread until tomorrow morning."
Not having been read.
"The stack of letters on the desk remained unread until she had time to sort through them."
The word unread is formed by adding the prefix un- to read to indicate that something has not been read yet. It entered English as a straightforward negation of the verb to read.