something i don't know in 2D array help me out in this

I have some doubts related to initializing 2D array. at many places, (websites like geeks for geeks) I saw this type of declaration used.

#include <iostream> 
using namespace std; 

const int MAX = 100; 

// Fills transpose of mat[N][N] in tr[N][N] 
void transpose(int mat[][MAX], int tr[][MAX], int N) 
{ 
    for (int i = 0; i < N; i++) 
        for (int j = 0; j < N; j++) 
            tr[i][j] = mat[j][i]; 
}