The bolded code is a conversion operator. (AKA cast operator)
It gives you a way to convert from your custom INT type to another type (in this case, int) without having to call a special conversion function explicitly.
For example, with the convert operator, this code will compile:
INT i(1234);
int i_2 = i; // this will implicitly call INT::operator int()
Without the convert operator, the above code won’t compile, and you would have to do something else to go from an INT to an int, such as:
INT i(1234);
int i_2 = i.a; // this wont compile because a is private