I THINK that is another syntax to invoke the constructor (from WString.h):
String(const char *cstr = "");
C++ has several initialization syntaxes: (), =, {}. The differences between them seem rather esoteric to me. But, I'm sure there's a reason for them that perhaps someone may be able to explain.
For simple initializations, I've found that several forms will work. The compiler seems to get more picky as the initializations get more complex. I usually end up playing with different forms until the compiler is happy. Like I said, there's probably some subtle difference.