When do I get this error?

The "Use a named parameter" error is thrown when JSLint encounters a access a property of the arguments object by numerical index. The following example adds two numbers. Since the function has no named parameters it uses the arguments object:

function add2() {
    "use strict";
    return arguments[0] + arguments[1];
}

Why do I get this error?

This error is raised to highlight potentially slow and potentially confusing code. Using the arguments object is slow. Many JavaScript engines will not actually create the object unless you reference it within a function. Running a benchmark in Chrome 30.0 reveals a 54% performance improvement when using named arguments.

As well as the performance issues, using the arguments object over named parameters harms the readability of your code. It is much easier to understand what a function is going to do, and what you should be passing it, when that information is available from the signature. To solve this error, simply use named function parameters where possible:

// This is much easier to understand
function add2(firstNumber, secondNumber) {
    "use strict";
    return firstNumber + secondNumber;
}

Note, however, that there are valid use cases for the arguments object. JSLint will only warn when you attempt to access a property of it by numeric index. The reason for this is that if you know the position of the argument in the list, there should be no reason you cannot give it an identifier in the function signature. Here's an example of a slightly more useful add function which uses the arguments object to allow the addition of any number of arguments:

/*jslint plusplus: true */
function add() {

    "use strict";

    var total = 0,
        i;

    for (i = 0; i < arguments.length; i++) {
        total += arguments[i];
    }

    return total;
}

This passes JSLint because we are no longer using a numeric index directly to access an argument. Even though i refers to a number JSLint will allow this to pass as it's a valid use case for the arguments object.