Complex sentence: The function of the underwriter is to protect the company's book of business from risks that they feel will make a loss and issue insurance policies at a premium that is commensurate with the exposure presented by a risk.
Split into simpler sentences: The function of the underwriter is to protect the company's book of business from risks that they feel will make a loss. In simple terms, it is the process of issuing insurance policies at a premium that is commensurate with the exposure presented by a risk.