It's when people box up all the unwanted gifts from Christmas, and put them in storage so they can regift them next year. LOL
In Britain, it was a custom for tradesmen to collect "Christmas boxes" of money or presents on the first weekday after Christmas as thanks for good service throughout the year. This is mentioned in Samuel Pepys' diary entry for 19 December 1663. This custom is linked to an older English tradition: Since they would have to wait on their masters on Christmas Day, the servants of the wealthy were allowed the next day to visit their families. The employers would give each servant a box to take home containing gifts and bonuses, and sometimes leftover food.
In England and Australia people either give gifts in boxes today or exchange gifts using boxes. I think that is where the name "boxing day" comes from. Why don't you google this or look on Wikipedia for more?