Referral link is a special link with UTM parameters added at the end. It looks something like this: https://www.onlinestore.com/join?promo_code=INFLU67478
The parameters make it possible to track the traffic and conversions that came from a specific referral – like an influencer blog or Instagram, for example. If you do this type of partnerships, you should be giving out individual referral links to all influencers and referral partners to measure their results.
This is different from discount code, which is just INFLUENCER10OFF, for example, that needs to be applied at checkout.