萌新初学C#求大佬指教下这道题

static void Main(string[] args)
{
int sum = 0;
for (int i = 1; i <= 15; i++)
{
sum += Factorial(i);
}
Console.WriteLine(sum.ToString());
Console.ReadLine();
}
public static int Factorial(int num)
{
if (num == 0)
{
return 1;
}
return num * Factorial(num - 1);
}

请问你这个if（i%3==0）是什么意思