An Incontestability Clause is a clause in the insurance policy which states that the insurance company will not deny a claim on the basis that you misrepresented yourself on the application for insurance. Usually, the policy must be in effect for two years before the Incontestability Clause applies.
If your policy does not have an Incontestability Clause, then the insurance company can ALWAYS deny a claim if it shows that you made misrepresentations on the application, EVEN if you have been paying premiums for years. However, the insurance company must reimburse all of your premium payments.