帮助我理解这个c#代码

C#3.0开始时的这段代码：面向对象编程简介

这是一个程序，让用户在多行文本框中输入几句话，然后计算每个字母在该文本中出现的次数

  private const int MAXLETTERS = 26;            // Symbolic constants
  private const int MAXCHARS = MAXLETTERS - 1;
  private const int LETTERA = 65;

private void btnCalc_Click(object sender, EventArgs e)
  {
    char oneLetter;
    int index;
    int i;
    int length;
    int[] count = new int[MAXLETTERS];
    string input;
    string buff;
    length = txtInput.Text.Length;
    if (length == 0)    // Anything to count??
    {
      MessageBox.Show("You need to enter some text.", "Missing Input");
      txtInput.Focus();
      return;
    }
    input = txtInput.Text;
    input = input.ToUpper();

    for (i = 0; i < input.Length; i++)    // Examine all letters.
    {
      oneLetter = input[i];               // Get a character
      index = oneLetter - LETTERA;        // Make into an index
      if (index < 0 || index > MAXCHARS)  // A letter??
        continue;                         // Nope.
      count[index]++;                     // Yep.
    }
    for (i = 0; i < MAXLETTERS; i++)
    {
      buff = string.Format("{0, 4} {1,20}[{2}]", (char)(i + LETTERA)," ",count[i]);
      lstOutput.Items.Add(buff);
    }
  }

我不理解这条线

 count[index]++;

和这行代码

buff = string.Format("{0, 4} {1,20}[{2}]", (char)(i + LETTERA)," ",count[i]);