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);