I haven't done one in ages! I'll PM you with my details.
It's a bunch of coupons put in an envelope and sent out in the mail. You get the envelope, open it up, take out any that have expired, take what you can use, then add back the value in new coupons that you took out or more. Then you send it out to the next person listed on the instruction sheet.
It's great fun, and a way to get coupons you never see in your area and a way to pass on good coupons you have but can't use...