A "barn raising" is a communal activity where a community of people gets together over the course of a few days to construct a barn on behalf of one of its member families.
In preindustrial agricultural societies, a barn was a crucial edifice for each farming family to have, in order to shelter livestock and store tools, hay, grain. However, most ordinary families would not have enough resources or manpower to construct a barn all by themselves. Thus, in many such societies, the whole community would come together to supply materials and construct a barn together whenever a member family required one (whether due to the formation of a new family following a marriage, or due to the destruction of a barn by fire or other disaster), with the understanding that if they or their offspring one day needed a new barn, the favor would be returned.
Barn raisings were typically festive occasions with ample food, drink, dancing, and celebration, especially once the barn was completed. The custom of barn raising has largely died out in the advanced Western world, but is still practiced by deliberately anachronistic groups such as Amish and Mennonite communities in North America.