(Note: The standard syntax is simply curl file:///path/to/file . The word "url" in your string is likely part of a variable name, a typo in the exploit script, or a flag meant to be parsed by a specific tool.)
This string is often associated with attacks. curl-url-file-3A-2F-2F-2F
In this command, curl would read the contents of the local /etc/passwd file. a typo in the exploit script
curl file:///
Interpreted meaning: