Can somebody please help me understand this JS code

function merge(arr, l, m, r)
{
    console.log(`L = ${l}, m = ${m} and r = ${r}`)
    var n1 = m - l + 1;
    var n2 = r - m;

    // Create temp arrays
    var L = new Array(n1);
    var R = new Array(n2);

    // Copy data to temp arrays L[] and R[]
    for (var i = 0; i < n1; i++)
        L[i] = arr[l + i];
    for (var j = 0; j < n2; j++)
        R[j] = arr[m + 1 + j];


    // Merge the temp arrays back into arr[l..r]

    // Initial index of first subarray
    var i = 0;

    // Initial index of second subarray
    var j = 0;

    // Initial index of merged subarray
    var k = l;

    while (i < n1 && j < n2) {
        if (L[i] <= R[j]) {
            arr[k] = L[i];
            i++;
        }
        else {
            arr[k] = R[j];  
            j++;
        }
        k++;
    }

    // Copy the remaining elements of
    // L[], if there are any
    while (i < n1) {
        arr[k] = L[i];
        i++;
        k++;
    }

    // Copy the remaining elements of
    // R[], if there are any
    while (j < n2) {
        arr[k] = R[j];
        j++;
        k++;
    }
}

// l is for left index and r is
// right index of the sub-array
// of arr to be sorted */
function mergeSort(arr,l, r){
    if(l>=r){
        return;//returns recursively
    }
    var m =l+ parseInt((r-l)/2);
    mergeSort(arr,l,m);
    mergeSort(arr,m+1,r);
    merge(arr,l,m,r);
}

// UTILITY FUNCTIONS
// Function to print an array
function printArray( A, size)
{
    for (var i = 0; i < size; i++)
    document.write( A[i] + " ");
}


var arr = [ 12, 11, 13, 5, 6, 7 ];
    var arr_size = arr.length;

    document.write( "Given array is <br>");
    printArray(arr, arr_size);

    mergeSort(arr, 0, arr_size - 1);

    document.write( "<br>Sorted array is <br>");
    printArray(arr, arr_size);