Assigning one array to another array c++

Hello I am beginner in c++ , can someone explain to me
this

char
            a[]="Hello";


char b[]=a; // is not legal
            

whereas,

char
            a[]="Hello";

char* b=a; // is legal
            

If a array
cannot be copied or assigned to another array , why is it so that it is possible to be
passed as a parameter , where a copy of the value passed is always made in the method

void copy(char[]
            a){....}

char[]
            a="Hello";

copy(a);