Nautilus applies emblems automatically to icons for files that are read or write only and to icons that are links (aliases) to other files or folders.
The default emblems change depending on your theme. The ones shown here are for two of the Nautilus themes, but if you have a custom theme, your emblems may be different.
Read only: You have permission to view this file or folder, but you can't modify it.
You can't view or modify this file or folder.
A link icon denotes a file that contains no content of its own but links to a file or folder located elsewhere on the computer. Clicking this icon opens the linked file or folder. (A link is similar to a shortcut in Windows or an alias in the Mac OS.)