"val:string" what does mean!?

I have no idea what this means at all to put it into context:

package
{
public class Name
{
private var firstName:String;
private var lastName:String;

function Name()
{

}

public function SetFirstName(val:String):void
{
firstName = val;
}

public function SetLastName(val:String):void
{
lastName = val;
}

public function GetFullName():String
{
return firstName + " " + lastName;
}
}
}