The traditional gift to the UK Poet Laureate, what is a "butt of sack"?

No longer the gift of the monarch, since 1984 the Laureate’s sherry has been a gift from the Sherry Producers of Spain as a goodwill gesture to celebrate the cultural and trading links between Britain and Spain. The Poet Laureates are invited to Jerez to select their sherry from a wide variety on offer, and usually design their own labels.