A more thorough explanation:

Cestui Que Vie refers to the person whose life is used to measure the duration of a trust, gift, estate, or insurance contract. This means that the length of time that the trust, gift, estate, or insurance contract lasts is based on how long the person whose life is being measured lives.

For example, if John creates a trust for the benefit of his daughter, Jane, for the duration of Jane's life, then Jane is the cestui que vie. The trust will last as long as Jane is alive.

Another example is if Sarah takes out a life insurance policy that will pay out to her husband, Tom, for the duration of Sarah's life. In this case, Sarah is the cestui que vie. The life insurance policy will pay out to Tom as long as Sarah is alive.