A more thorough explanation:

A secondary insurer is an insurance company that is responsible for settling any part of a claim that is not covered by the primary insurer. The primary insurer is the first insurance company that is contractually committed to settling a claim up to the applicable policy limit before any other insurer becomes liable for any part of the same claim.

For example, if a person has two insurance policies that cover the same loss, the primary insurer will pay up to the policy limit, and the secondary insurer will pay any remaining amount that exceeds the primary insurer's policy limit.

Another example is when a person has health insurance through their employer and also has Medicare. The employer's health insurance is the primary insurer, and Medicare is the secondary insurer. The primary insurer pays first, and the secondary insurer pays any remaining amount that is not covered by the primary insurer.