For instance, if you are married and want to buy life insurance on your husband, there must exist an insurable interest between the two of you.
Insurable interest means you stand to lose financially if the insured person were to die, or you rely on the insured person for financial support.
Insurable interest does exist between spouses, siblings, parents and their children, relatives, business partners, etc.
It's a way of making sure people don't buy life insurance on random people who are about to die, in order to collect on the life insurance.
Here's an article explaining What is Insurable Interest?.