A more thorough explanation:

Definition: The rule against perpetuities is a common-law rule that prohibits a grant of an estate unless the interest must vest, if at all, no later than 21 years (plus a period of gestation to cover a posthumous birth) after the death of some person alive when the interest was created. The purpose of the rule is to limit the time that title to property could be suspended out of commerce because there was no owner who had title to the property and who could sell it or exercise other aspects of ownership. If the terms of the contract or gift exceeded the time limits of the rule, the gift or transaction was void.

Examples:

• John creates a trust that will give his property to his grandchildren, but only if they graduate from college. The youngest grandchild is currently 10 years old. This trust violates the rule against perpetuities because it is possible that the youngest grandchild may not graduate from college until more than 21 years after the death of John.
• Emily creates a will that leaves her property to her great-grandchildren. The youngest great-grandchild is currently 5 years old. This will violates the rule against perpetuities because it is possible that the youngest great-grandchild may not be born until after 21 years after the death of Emily.

These examples illustrate how the rule against perpetuities limits the time that title to property can be suspended out of commerce. In both cases, the terms of the gift exceed the time limits of the rule, making the gift or transaction void.