A CD is a time deposit: you agree to leave your money in place for a fixed term in exchange for a guaranteed rate. If you take the money out before the maturity date, the bank generally charges an early-withdrawal penalty.
That penalty is usually expressed as a set number of months of interest — for example, 90 days of interest on a short-term CD or six months or more on a longer one. If you withdraw early enough that you haven't yet earned that much interest, the penalty can reduce your original principal. A few products offer “no-penalty” CDs that waive this charge. If you think you may need the funds sooner, consider a shorter term, a no-penalty CD, or a CD ladder so part of your money matures regularly.