Not sure what th is, but ref is the referrer’s ID, which gives the referrer a referral / affiliate bonus if you purchase the item using that link. In theory it’s not a bad way to support the referrer and it’s not linked to you as an individual personally. You can remove it of course if you feel like they don’t deserve the money for referring you to a deal. In the end ref or no ref the price of the item remains the same for you.