Estoppel is a legal term that means preclusion. Estopped = precluded.
There are different types of “estoppel” in standard legal jargon. Equitable estoppel and judicial estoppel quickly come to mind. My favorite is the “mend the hold” estoppel doctrine, which draws its name from centuries-old wrestling terminology.
Different estoppel doctrines will have specific applications. But the basic premise is that because of some prior action or inaction, a party is “estopped” from taking some future action or position.
In the insurance law context, estoppel refers to when the insurance company is estopped from enforcing policy terms. This could mean that an insurer cannot deny coverage based on a particular policy term(s) or even that the company has to pay more than the policy’s limits.