A read-only file is any file with the read-only attribute
turned on. Some common files that are read-only by default in Windows include boot.ini, io.sys, msdos.sys and others.
A file with the read-only attribute turned on can be opened and accessed normally by your computer, but you will not be able to make changes to it. In other words, "writing" to the file is disabled.
Read-only files can be deleted and moved, but Windows will prompt you with a special dialog box asking you to confirm that you want to move or delete the read-only file.
Examples: "I have an important spreadsheet document that I share with other users at
work. If any of the formulas were to be accidentally changed, the entire
spreadsheet would be useless. To help protect the spreadsheet, I set the
read-only attribute on the file. Now if anyone makes changes and then tries to
save them, they won't be able to."