Find the kth smallest element from 2 sorted arrays

I came across the solution for this problem:

I would like to know the concept behind selecting the low and high values?

int findKthSmallest(int arr1[],int size1,int arr2[],int size2,int k)
{
//check whether we have k element after combining both arrays.
assert(size1 + size2 >= k);
assert(k>0);
//find index low <= w < high such that w from arr1 and (k-w) from arr2, are the smallest k numbers of resultant.
 int low = max(0 , k - size2 ); //define start point
 int high = min(size1 , k);//define end point

while (low < high)
{
    int w = low + (high - low) /2;
    //Can we include arr1[w] in the k smallest numbers that we are finding?
    if ((w < size1) && (k-w > 0) && (arr1[w] < arr2[k-w-1]))
    {
        low = w + 1;
    }
    else if ((w> 0) && (k-w < size2 ) && (arr1[w-1] > arr2[k-w]))
    {
        //can  we discard arr1[w -l] from the set of k smallest numbers.
        high = w;
    }
    else
    {
        //Both the inequalities were false, break.
        low = w;
        break;
    }
}
if (low == 0)
{
    return arr2[k -1];
}
else if (low == k)
{
    return arr1[k-1];
}
else
{
    return max(arr1[low -1], arr2[k - low - 1]);
}
return 0;
}