When someone offers to do work or help out for free, they're engaging in volunteerism. Donating your time at a soup kitchen, animal shelter, or after school program is all part of volunteerism.
Any kind of volunteer-based community service is considered to be volunteerism, including coaching at the Boys and Girls Club or retirees reading books to preschoolers. When you volunteer, you're taking part in this tradition of helping others. A college application might even ask about volunteerism — it's a good opportunity to brag about all those hours planting trees in the city parks. Volunteerism originally referred to people voluntarily joining the military, from a Latin root meaning "of one's free will."