The CD is a "Certificate of Deposit", it's a way of investing your arcade points. You buy one and than so many days later (7 for a CD7, 14 for a CD14, etc...) and it matures. When it matures, you get the original money back plus what ever the interest rate was. For example, if you buy a 200 point CD7 than after a week you get something like 248 pionts.