Reward point is a Opencart feature and you can read on Opencart website how it works here: http://docs.opencart.com/en-gb/report/customer/reward/

A video link: https://www.youtube.com/watch?v=FrtnSFdAKPQ

Also a guide article by template monster: https://www.templatemonster.com/help/opencart-2-x-how-to-work-with-reward-points.html