To set up customer referrals, click Marketing > Customer referrals > set your criteria for what your customers will receive.
Here's how Referrals works.
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 one, pictured below
Once that 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.