This holiday known in Britain, Ireland, Canada, New Zealand, and Australia is sometimes also called St. Stephen's Day.
There have been many stories about what Boxing Day means. The most common story I've heard is from the 1900s where upper class British people would give gifts or money in a small "Christmas box" to thank their servants and service men and women (cleaners, paper boy, postman). This tradition still continues for some people today where they will donate their time or buy gifts to charities and for people who are less fortunate.
Some people have also said it started from the Church of England where churchgoers would put their monetary donations in a box many days before December 26th. On Boxing Day, this box would be opened and the money would be given to the poor.
Either way, Boxing Day is widely known as a day to share something with people who are less wealthy or those who serve us.
Most recently, however, Boxing Day has also become known as a big shopping day! This is the day that a lot of stores have big sales and offer doorbuster deals. It seems like stores just like to take every opportunity to attract some attention!