an add on is simply things you want after you have made your initial pledge. after you pledge for a reward you really want, you can add on more pledges or things from the add on category or from the stretch goals category buy simply adding the amount of of the item you want to your pledge.
if you pledged $50 for a reward and decide you want to add on another reward that is also $50, then add that extra $50 to your pledge. To do this, click the manage button next to your pledge amount on this campaign. After you're finished, then message Dan and Friends about what you added. this helps you to remember what you ordered, so at the end of the kickstarter, it will be easier to fill out the survey.