Common Good Credits have value and act like money because we agree to accept them as payment for our good and services, so they move from member to member to member as a token of exchange. When someone pays you in Common Good Credits, you can spend those funds — OR you can "cash them out" by transferring them to your bank account or exchanging them for cash.
You can cash out Common Good Credits that someone pays you, but any Common Good Credits you receive free, as Incentive Rewards (if your Common Good community is offering Incentive Rewards), must continue to circulate within the Common Good system. You cannot cash them out — you can only spend them.
You can help keep the Common Good system healthy by maintaining a positive Balance (most of the time) and treating your incentive rewards, if any, as a Credit Reserve.