From the rulebook:
"Players represent insurance companies who compete to insure bond issues sold by municipal governments. Just as in real life, one issuer may sell more than one bond issue, each of which might be insured by a different insurance company. The 36 different issuers have different probabilities of default, with riskier issues represented by dice faces with numerically higher values. Riskier issuers pay higher premiums to their insurance company, but also have a greater chance of defaulting and causing the insurer to pay a claim." Most money at the end wins.
The box claims the game is for ages 14 and up, but it's more straightforward than that. And "although knowledge of the market for financial guaranties is not necessary to win INSURE!, some familiarity with it and its specialized terminology may add to your fun."