A "silver pillow" refers to cask wine, or, more specifically, the bladder inside the box. This bladder is made from a silver coloured plastic foil material. After consuming the contents (goon), you can inflate the empty bladder and use it to sleep on.
Many have appreciated this handy built in pillow feature of cask wine, and the silver pillow has provided many a comfortable night's sleep to drunks all over the world.