C# newbie: how to know the meaning of a variable in a lambda expression

I'm new in a company, using C# (I'm used working with other languages), and I just encountered a Lambda expression in the code, so I'm trying to understand how this works, but the explanation on the Microsoft website is far from being clear:

static void Main(string[] args)  
{  
    string[] digits = { "zero", "one", "two", "three", "four", "five",   
            "six", "seven", "eight", "nine" };  

    Console.WriteLine("Example that uses a lambda expression:");  
    var shortDigits = digits.Where((digit, index) => digit.Length < index);  
    foreach (var sD in shortDigits)  
    {  
        Console.WriteLine(sD);  
    }  
}

When looking at the definition of "digits", I only see the content, but I can't understand why index is indeed the index of the string. Let me explain you what I mean, by replacing the name of that variable by something:

static void Main(string[] args)  
{  
    string[] digits = { "zero", "one", "two", "three", "four", "five",   
            "six", "seven", "eight", "nine" };  

    Console.WriteLine("Example that uses a lambda expression:");  
    var shortDigits = digits.Where((digit, something) => digit.Length < something);  
    foreach (var sD in shortDigits)  
    {  
        Console.WriteLine(sD);  
    }  
}

=> How can I know that the meaning of something is the index and not another property of strings[]?

Relevant question: can I add more to this list of variables (why/why not)?

static void Main(string[] args)  
{  
    string[] digits = { "zero", "one", "two", "three", "four", "five",   
            "six", "seven", "eight", "nine" };  

    Console.WriteLine("Example that uses a lambda expression:");  
    var shortDigits = digits.Where((digit, var1, var2, ...) => digit.Length < var1);  
    foreach (var sD in shortDigits)  
    {  
        Console.WriteLine(sD);  
    }  
}

=> This seems not to compile, there is a syntax error, with following message: Delete 'Func<string,bool>' does not take 3 arguments. Why bool? index is not a Boolean, but an integer. What am I missing?