Underwriters are insurance companies carrying the risk of an insurance policy. Sometimes it's the same company as the one selling the insurance policy. Sometimes it's a different company.
In our case, the underwriter our your contents and buildings policy is Casualty & General Insurance Company (Europe) Limited (CGICE). The underwriter of your Home Emergency and Legal Expenses policy (if you have it) is AmTrust Europe Limited.
To see the definitions for other commonly used insurance terms, check out our dictionary.