Halloween is a strange holiday.  It started several centuries ago in Europe.  It began as "Hallow Evening" meaning "holy night"   At that time November 1 was called "All Saints Day" and the people honored all the saints that did not have their own special day. The night before that, October 31, became a time when all the evil spirits were supposed to be active. (来源：专业英语学习网站 http://www.EnglishCN.com)

On "Hallow 'een", all the bad spirits came out to do mischief.

People would play all sorts of pranks on each other. Practical jokes, Put a cow in the neighbor's house, things like that.

Now it has evolved to a children's holiday.  Small children go "Trick or Treating"    They wear costumes, knock on doors, and yell, "Trick or Treat". People give them candy and treats.

The original idea was a form of friendly extortion.  "Give me a treat, or I'll play a trick on you." The threat was, something like, "I'll spill all the trash out of your trash can if you don't give me a treat." 

It's a lot of fun for children. They dress up like ghosts, witches, and such.

Adults sometimes "trick or treat" their friends too. Usually that means the "treat" will be a glass of wine or something.