A CD ladder splits your money across several CDs with staggered maturity dates — for example, equal amounts in 1-, 2-, 3-, 4- and 5-year CDs. Each year one CD matures, giving you access to a portion of your money without an early-withdrawal penalty. When it matures, you reinvest it into a new longest-term CD, so the ladder keeps rolling.
The point is to get most of the higher yield of long-term CDs while keeping regular access to cash. After the first few years, you hold all long-term CDs (which typically pay more) but still have one maturing every year. It's a simple, low-risk way to manage cash you won't need all at once.