A more thorough explanation:

A substituted executor is a person who is named as an alternate executor in a will. If the original executor is unable or unwilling to carry out their duties, the substituted executor will step in to fulfill the provisions of the will.

For example, if a testator names their spouse as the executor of their will, but the spouse passes away before the testator, the testator may name their adult child as the substituted executor.

The substituted executor is only called upon if the original executor cannot fulfill their duties. This ensures that the testator's wishes are carried out even if the original executor is unable to do so.