What is meant by trick or treat?

It’s a fun Halloween tradition, mostly for kids. On October 31st, children dress up in costumes. They walk around their neighbourhood and knock on doors. When the door opens, they shout, "Trick or treat!" This is their way of asking for candy. The 'treat' is the candy they receive. The 'trick' is a playful, old-fashioned threat of a harmless prank if they don't get one. Honestly, it's all about getting the candy these days