Understand two lines simple code

static void Main(String[] args)
{
    var nk = Console.ReadLine().Split(' ').Select(Int32.Parse)
                    .ToArray();
    int n = nk[0];
    int k = nk[1];
    var arr = Console.ReadLine().Split(' ').Select(Int32.Parse)
                     .ToArray();
        
    var count = 0;
    var counts = new int[k];
    for (var i = 0; i < n; i++)
    {
        // the idea is that if (a1 + a2) % k == 0
        // then (a1 % k + a2 % k) should be equal to k (or 0)
        var bucket = arr[i] % k;
        // adding all new combinations with arr[i] to the count
        // also handling bucket == 0 with % k here
        count += counts[(k - bucket) % k];
        counts[bucket]++;
    }
        
    Console.WriteLine(count);
}

count += counts[(k - bucket) % k];
       counts[bucket]++;