Trick-or-treating is a traditional Halloween custom for children and adults in some countries. During the evening of Halloween, on October 31, people in costumes travel from house to house, asking for treats with the phrase "trick or treat". The "treat" is some form of confectionery, usually candy/sweets, although in some cultures money is given instead. The "trick" refers to a threat, usually idle, to perform mischief on the resident(s) or their property if no treat is given.

Source: Wikipedia