It means "put a bunch of holly branches all over the walls." "Deck" means to dress in, as in, "she was decked in satin and diamonds." "The halls" means, specifically, the hallways and entrance hall...
The music to Deck the Halls is believed to Welsh in origin and was reputed to have come from a tune called "Nos Galan" dating back to the sixteenth century. In the eighteenth century Mozart used the...