Basically you can use some of all of your points to redeem free items. Products will have various points values you can add multiple items or save points to redeem your goal product. Just make sure you have at least 1 paid item in your Cart!
To do the redemption please log into your account, Rewards Program link in the bottom left corner (on a desktop), or in the main menu on your mobile, then look for the Spend Points button, or take the opportunity when checking out. From there you should be able to nab yourself some skin Woohoo happiness :)