A explanatory video from Queenie Tan - Money Magazine, on how cashback works.
Some retailers (the merchants), that you purchase from are willing to pay “commission” to affiliate networks and other website publishers to gain sales. As a website publisher themselves, the cashback website is rebating/refunding a portion of their commissions (that they've made from the merchants) back to their customers (you).
For example Kiwiwallet are currently offering 5% cashback for purchases from Myprotein.
Since the purchase price from Myprotein will still be the same regardless of whether a commission is paid, it is advised to use a cashback/commission-refund site to make your purchases as often as possible.
Note: Whilst examples on this page are using Kiwiwallet, you should do your own research to determine which provider is best for you and is offering the best rate for the store you are planning to purchase from.
Currently ChoiceCheapies lists cashback offers from 3 cashback sites: