The meaning of "declarations mirror use" is that the programmer can use the declared variable in the exact same way as being declared (by applying the exact same operators):
int **p; // declares "p" as a pointer to a pointer to int
int x = **p; // the "**p" expression mirrors the declaration and its type is "int"
This is true even for functions:
int *f(int a); // declares a function which takes an int and returns a pointer to int
int res = *f(3); // the "*f(3)" expression mirrors the declaration and its type is "int"
Arrays are obvious when looked that way:
int arr[5][5];
int elem = arr[1][2]; // the "arr[1][2]" expression mirrors the declaration
The advantage is that you don't introduce any new operators or syntactic forms which would only be used in declarations. This means a smaller set of tokens and a smaller set of syntactic forms. Being able to parse an expression like
(*arr[i])(42)
means that you can use pretty much the same machinery to parse
