A CD ladder is an investment strategy, the uses money in a similar way of diversification. A CD ladder differs because you aren’t diversifying based on the type of investment, you are diversifying based on time.
Here is an example…
You have a sum of money that you want to invest in a CD or certificate of deposit. This is more or less a loan you give the bank with an interest returned to you that is larger than a savings or checking account.
Instead of investing this all at once, you split the money into equal parts and invest each of these to stagger the maturity rates.
The CD ladder strategy decreases both interest rates and the risk associated with a single lump sum investment.