I really don't get this. I don't understand how is it that the .prev() works. Could someone please explain this to me?

Hey Andrea, A little late to the party. Lets see if I can add to the above clarification. It took me a while to get it because in the video he was explaining why use .prev() while in the app.js file and not referencing the html file.

So lets say your page loads and runs your script up to this point...

 //1. Hide Spoiler
 $(".spoiler span").hide();

 //2. Add a button
 $(".spoiler").append( "<button>Reveal Spoiler!</button>" );

After the above script runs, your .spoiler HTML now looks like this:

 <p class="spoiler">
      <!--Spoiler:-->
      <span>Darth Vader is Luke Skywalker's Father! Noooooooooooo!</span>
      <button>Reveal Spoiler</button>
 </p>

 <p class="spoiler">
      <!--Spoiler-->
      <span>Luke and Leia are siblings. Ew.</span>
      <button>Reveal Spoiler</button>
 </p>

Now we want to only show one span at a time when the button is clicked. As in the example, we use the following code:

 $(this).prev().show();
 //Translation: Show the previous sibling of THIS [the .spoiler button]. 
 //What is the previous sibling of the button as seen in the HTML above? The span)

I hope that adds to the what is already here. For me it was hard to see what .prev() was doing by only referencing the app.js file.

Thank you Stanford for this explaination! Now I understand very clearly how it works!

this is really helpful thanks guys!