Stuck on this test. Do not really know were to begin. Looks like they already have a string parameter named person.

from the swift ebook:

If you want users of your function to provide parameter names when they call your function, define an external parameter name for each parameter, in addition to the local parameter name. You write an external parameter name before the local parameter name it supports, separated by a space:

func someFunction(externalParameterName localParameterName: Int) {
    // function body goes here, and can use localParameterName
    // to refer to the argument value for that parameter
}

If you want to provide an external parameter name for a function parameter, and the local parameter name is already an appropriate name to use, you do not need to write the same name twice for that parameter. Instead, write the name once, and prefix the name with a hash symbol (#). This tells Swift to use that name as both the local parameter name and the external parameter name.

right now it just has an internal parameter. you can add an external parameter with the same name as the internal by adding a # in front of the parameter name

func greeting(#person: String) {
    println("Hello \(person)")
}

by adding an external parameter, you mandate that the parameter name must be provided in the function call.

before adding the external parameter the function call would have looked like this:

greeting("Stone")

After adding the external parameter name you have to provide the parameter name in the function call

greeting(person: "Stone")

see the swift ebook for reference