How to interpret "operator const char*()" in operator overloading?

How to interpret "operator const char*()" in operator overloading?

I was looking at one of the implementation of String class and noticed the
following overloaded == operator.
enter code hereString f = "something"; enter code hereString g =
"somethingelse"; enter code hereif (f == g) enter code herecout <<
"Strings are equal." << endl;
enter code herebool operator==(String sString) { return strcmp(operator
const char*(), (const char*)sString) == 0; }
I understood most of the part except operator const char*() what exactly
its been used for? I have very basic knowledge of operator overloading ,
can someone please throw some more light on this?