The "Bring a friend referral code" gives your friend $20 off their first month's bill and you receive a $20 credit that can be applied to your monthly bill. Win Win! Keep in mind you can build up to 12 credits at a time, but this doesn't mean you're limited to earning 12 credits per year. As you use credits each month you can build them back up to 12 again. So you can potentially earn up to 24 credits per year!
It's actually REALLY EASY to build credits too. I've already earned 12 credits ($240 total) since late November 2022. I've done this just by posting my friend code on Reddit and here on Visible's community forum.