What The Heck Is Boxing Day?

I thought Kwanza was confusing - but Boxing Day beats it by a mile! I looked it up[ on Wikipedia and even they don't know. It started in the 1700's I guess and is explained as floowos - "A present or gratuity given at Christmas: in Great Britain, usually confined to gratuities given to those who are supposed to have a vague claim upon the donor for services rendered to him as one of the general public by whom they are employed and paid, or as a customer of their legal employer; the undefined theory being that as they have done offices for this person, for which he has not directly paid them, some direct acknowledgement is becoming at Christmas."

Well, WTF does THAT mean? And why does every one get the day off? And why is it a big shopping day? Apparently there are state holidays some places
in the USA - but they don't call it Boxing Day

This is all too loosely structured people! Is this like Senior skip out day? You know, you're not supposed to do it, but everyone knows you're gonna do it, so it's OK that you do it - even though you're not supposed to do it? Is it like that?