Volunteer work is done by choice and you do not get paid for it. Many people do volunteer work for idealistic, humanitarian, political or religious reasons, to make the world a better place.
There are a lot of volunteers helping socially disadvantaged people. They go shopping for older people, organize homeless cafes (similar to soup kitchens), or help children from disadvantaged families with their homework. Many volunteers also teach German to refugees or help them to deal with bureaucracy.
Others are politically engaged and devote themselves, for example, to human rights campaigns, organizing information events, raising political grievances or writing protest letters. There are also lots of people working in the area of environment protection. They raise awareness of environmental problems, protect birds and look after sick animals.
You'll find noticeboards on the internet and at neighborhood centers about how to get involved in volunteer work.