Voluntary is an adjective that describes something you do because you want to, without being influenced or forced into it. Usually a voluntary act is something you consciously choose to do, like going into work even on a snow day.
Voluntary comes from the Latin word voluntarius, which means "of one's free will." Voluntary can also describe a service given without any payment expected in return. For example, if your work for the community theater is voluntary, that means that you don't get paid for your time. If you're offering your voluntary help in this way, you might also be called a volunteer, someone who offers free help to assist others.