To set up customer referrals, click Marketing > Customer referrals > set your criteria for what your customers will receive.
Here's how Referrals work:
After the referral feature is turned on, an existing customer of yours can find a referral link to share with their friends from either their mobile app or desktop account
After a new customer signs up using this special link, that new customer will get rewarded with whatever you've listed on line 1 pictured below
Once the new customer spends what you've listed on line 2, ($20 in the example above), the original customer will receive whatever you've listed on line 3.
When sharing links with friends, customers will be able to see a preview of what they stand to gain if their friend spends the required amount.
For more views on setup and how customers interact with this feature, see this video.
For a deeper dive on this feature, see this video that details all of its facets.