This takes the next step in a Simple Item. With add-ons you can add basic variances like Color or Sizes. These can be the same $ amount or you can add an addition $ to the base price. For example if you want to charge $2.00 more for an X-Large. Simple add-Ons will not take into account different SKU, Wieghts, Images. etc. You would use A Variable Product with item specific Variations.