Freeganism wiki says

Freeganism is an anti-consumerist lifestyle whereby people employ alternative living strategies based on "limited participation in the conventional economy and minimal consumption of resources". Freegans "embrace community, generosity, social concern, freedom, cooperation, and sharing in opposition to a society based on materialism, moral apathy, competition, conformity, and greed." The lifestyle involves salvaging discarded, unspoiled food from supermarket dumpsters, known as dumpster diving. Freegans salvage the food for political reasons, rather than out of need.
The word "freegan" is a portmanteau of "free" and "vegan". Freeganism started in the mid 1990s, out of the antiglobalization and environmentalist movements. The movement also has elements of Diggers, an anarchist street theater group based in Haight-Ashbury in San Francisco in the 1960s, that gave away rescued food.

Many freegans get free food by pulling it out of the trash, a practice commonly nicknamed dumpster diving in North America and skipping or bin diving in the UK, as well as bin raiding. Freegans find food in the garbage of restaurants, grocery stores, and other food related industries, which they assert allows them to avoid spending money on products that exploit the world's resources, contribute to urban sprawl, treat workers unfairly, or disregard animal rights. By foraging, they believe they are keeping perfectly edible food from adding to landfill clutter and that can feed people and animals who might otherwise go hungry.
Dumpster diving is not, however, limited to rummaging for food. Many divers search for anything that can be recycled or reused, from accessories to power tools in need of small repairs. Some divers collect aluminum cans, which they can then sell for a small profit. And though their name suggests much more of a physical aspect of this process, divers usually do not actually get into dumpsters. Often, these people have all sorts of equipment such as some sort of longer pole that they use to move items already in the dumpster around.