The key to making CDs work for you is knowing when they mature. A CD matures on the date specified in the account agreement. At that point, you can cash out the CD and get your money plus interest, or you can renew the CD for another term.
The biggest risk with a CD is that you lose free access to your money if you need it before the CD matures. If you need to withdraw money early, you may be able to do so, but you'll typically pay a penalty.
The specific rules vary depending on the bank and the type of CD, but early withdrawal penalties can be equal to or exceed several months' worth of interest.
The best way to avoid these penalties is to only invest money in a CD that you know you won't need for the full length of the CD term. That way, even if an emergency comes up, you can leave your money untouched and continue to earn interest without having to worry about paying any penalties.