Is there some definition about risk sharing?
Captive insurance is an insurance for purpose of insuring the owners of the said insurance. It’s a kind of self-insurance where multiple agents pool their capital together to insure themselves. So the term in that sentence refers to the pool of resources for insurance.

It’s called captive because the modern concept of this kind of insurance was first applied to a company that wanted to create insurance for its subsidiary mines that were referred to as captives and somehow the term catch on.