A more thorough explanation:

Definition: Prayer in aid is a legal term used to describe a plea made by a life tenant or someone who holds less than a fee simple to bring in another person who holds an interest in the estate, such as a reversioner or remainderman, to help defend the title.

Example: Let's say that John owns a piece of land, but he only has a life estate in it, which means he can use it during his lifetime but cannot sell it or pass it on to his heirs. If someone challenges John's ownership of the land, he can make a prayer in aid to bring in the person who holds the reversionary interest in the land, such as the person who will inherit it after John's death, to help defend his title.

Explanation: This example illustrates how prayer in aid works in practice. John, as a life tenant, does not have full ownership of the land, so he can bring in the reversioner to help him defend his title if someone challenges it. The reversioner has a future interest in the land, so it is in their best interest to help protect the title.