A more thorough explanation:

Definition: Destructibility of contingent remainders is a legal doctrine that requires a future interest to vest by the time it is supposed to become possessory, or else it will be destroyed, and the interest will revert to the grantor. This doctrine can be avoided by using trustees to preserve contingent remainders. However, this doctrine has been abolished in most American jurisdictions, and the abolishing statutes are commonly known as anti-destructibility statutes.

Example: Suppose a landowner grants a piece of land to A for life, then to B's children if B has any, and if not, to C. If B does not have any children, then the contingent remainder in favor of B's children will be destroyed, and the interest will revert to the grantor. However, if the landowner had used a trustee to preserve the contingent remainder, then the trustee could hold the interest until it vests.

Explanation: This example illustrates how the destructibility of contingent remainders works. If the future interest does not vest by the time it is supposed to become possessory, then it will be destroyed, and the interest will revert to the grantor. However, using a trustee to preserve the interest can avoid this result.