This song probably dates back to the 16th century though its origins are obscure. We do know that it is from England and was a favorite of carolers who went from door to door at Christmastime, soliciting food and drink in exchange for their harmony.
In the song, the words tell the story of a group of carolers that go to a house where nobody is home. So what? Well, if there’s nobody home to sing for, the singers won’t make any money, and so can’t afford to buy meat or drink.