A more thorough explanation:

An executory interest is a type of future interest in a transferee that can either divest the prior estate or spring out of the grantor to become possessory. These interests are non-vested and are subject to the Rules Against Perpetuities.

There are two types of executory interests:

• Shifting Executory Interest: This type of interest divests some interests in another transferee prior to its natural expiration, thereby cutting short the prior estate. However, many shifting executory interests violate the Rules Against Perpetuities.
• Springing Executory Interest: This type of interest divests the transferor in the future and has a gap of time. If there is any time not accounted for, the estate reverts back to the original owner as a reversion.

A fee simple subject to executory interest is a type of defeasible fee in which, on the happening of a stated event, automatically divests in favor of a third party that is not the transferor.

One example of an executory interest is a transfer of property from A to B, but with the condition that if B ever sells the property, it will automatically transfer to C. In this case, C holds a shifting executory interest because it divests B of their interest in the property if they sell it.

Another example is a transfer of property from A to B, but with the condition that B will only gain possession of the property once they turn 21 years old. In this case, B holds a springing executory interest because it will only become possessory in the future and has a gap of time.

These examples illustrate how an executory interest can either divest a prior estate or spring out of the grantor to become possessory in the future.