A more thorough explanation:

A substitutional gift is a type of testamentary gift where the testator leaves a gift to one person in place of another who is unable to take under the will for some reason. This type of gift is also known as a substitute gift.

For example, if a testator leaves a gift to their child, but the child predeceases them, the testator may name the child's child (i.e. the testator's grandchild) as the substitute recipient of the gift.

Another example is if a testator leaves a gift to their friend, but the friend is unable to take the gift due to death or incapacity, the testator may name another friend as the substitute recipient of the gift.

These examples illustrate how a substitutional gift allows the testator to ensure that their intended gift goes to someone they care about, even if the original recipient is unable to receive it.