This basically means one of two things.

You can advertise your game outside of BYOND, with a referral link. New players will see your ad, (hopefully) like what they see and play, signing up to BYOND in the process. If those people stay on and eventually become members, you get $5 for referring them. This is actually quite neat, if a little under-used.

You can (with a little prior discussion) get subscriptions enabled for your game, and provide some in-game benefits to subscribers through BYOND's subscription system. BYOND takes 10% of the subscription amount as commission for handling things like credit card charges, linking the subscription to the account via CheckPassport() and so on. Members who subscribe get that 10% knocked off the price, meaning members are a little more likely to subscribe. Generally speaking the games enabled for subscription have to be fairly free of copyright and legal issues, as BYOND is handling money on your behalf, hence a little prior discussion.