Voluntary means an action taken exercising free will, without compulsion, without legal obligation, without expectation of return or profit.

In insurance terms you will see voluntary referencing liability - under personal property policies there is coverage for "goodwill" type claims. You may not be truly negligent and therefore not liable. The insurance provides for small amounts to be paid out to keep costs down and the your neighbours happy. An example is when your dog is accused of destroying the neighbour's prize rose bushes by digging them up.

Voluntary can also reference the markets that are available to you, the consumer. If insurance is government-run then you do not have a choice in your provider. If it is an open or voluntary marketplace then you pick your company.