Can anyone explain the concept of trick or treat?

You dress up and walk around your neighborhood going door to door to "scare your neighbors, " Traditionally only the little bitty kids favored the non scary costumes but now it's a free for all. You say "trick or treat" when someone opens the door. Mostly they give you candy or rarely some small toy for little kids. You don't go to the houses that have the porch lights off so everyone who opens the door gives you a treat. But theoretically if the home owner doesn't give you a treat then you do something bad or mischievous like scare them or TP their house