When you mouse-over a response in the activity feed, you will notice a ★ (star) button appears just above the sentiment and below the image, sometimes with a number inside of it:
Clicking the star has a dual purpose. First and foremost, a starred response will always be visible under the ★ activity feed tab filter. When you see ideas you want to return to later, to respond to, for use in a paper or to discuss in class, star it and you can quickly return to it later, like a bookmark.
In addition, Ponder aggregates star counts on responses for each group, so you can see the responses your groupmates found reason to save. It's a little bit like clicking "thumbs up" except that you don't want to do that indiscriminately, or your bookmark collection will get over-filled. The count is indicated in the activity summary at the top of the nugget and on the star button itself.
Once you click the star button for a response, the number on the star will be incremented for everyone in the group. You can always un-star it, which will remove the response from your star filter and decrement the star count.