How to use .NET Action to execute a method with unknown number of parameters?

I want to execute some operations on a worker thread while displaying a progress bar to the user. I've created a class

public class ProgressBar
{
   public void StartAsyncTask(Action action)
   {
      Task t = new Task(action);
      t.start();
   }
}

I found out that I can send any method to the StartAsyncTask in the following way:

  ProgressBar pb = new ProgressBar();
  pb.StartAsyncTask( () => DoSomething(15, "something"));

  public void DoSomething(int i, string s)
   {
      //do something
   }

First of all, I can't seem to understand what is and how is lambda expression - () => - translated and how is the Action object passed a delegate with an unknown number of parameters.

I would like to use a BackgroundWorker with my ProgressBar but in this case I would need to invoke the action. So something like this:

void m_backgroundWorker_DoWork(object sender, DoWorkEventArgs e)
{
    Action action = e.Argument as Action; //same action here passed through EventArgs
   //but already in a worker thread so no need for the Task object 

   //and now i need to somehow invoke the action object but i don't know what the parameters are. 

   action.Invoke( ? ); 
}

How is it possible in the first example to execute the action without knowing the parameters in StartAsyncTask(Action action) method?

Why do I need to know the parameters when invoking the action in this case?

Everything about how/why/when to use "Action" is pretty unclear to me even if I read MSDN documentation and some other threads here. Any information on this will help me.