What is the meaning of ToString("X2")?

I'm studying MD5 encryption, and have found this code using Google:

public string CalculateMD5Hash(string input)
{

    // Primeiro passo, calcular o MD5 hash a partir da string
    MD5 md5 = System.Security.Cryptography.MD5.Create();
    byte[] inputBytes = System.Text.Encoding.ASCII.GetBytes(input);
    byte[] hash = md5.ComputeHash(inputBytes);

    // Segundo passo, converter o array de bytes em uma string haxadecimal
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < hash.Length; i++)
    {
        sb.Append(hash[i].ToString("X2"));
    }
    return sb.ToString();
}

Why does it use ToString("X2")? How is it different from normal ToString?