Let's say you're planning a vacation abroad and are booking a package with a tour operator. The operator will typically share with you a document that covers everything that is included in the package, as well as any additions that you may have to pay for.
A benefit illustration works in the same way. It has complete details about your policy journey and details of what you will get from your investment: from coverage to premiums, to duration, to benefits. It is your handbook for anything and everything about your policy, so you can refer to its contents whenever you are looking for more information.