Nasty and plastic based “food”, imagine if McDonald’s had a plastic parody with chemicals injected into it to make it taste more appealing at first bite but hours later your stomach hurts and you probably die from eating this garbage.
Refers to popular ballad sung by bards in Westeros (Game of Thrones setting) which narrates the annihilation of Lord Castamere by Tywin Lannister. More in general, the expression is used as a warning or a threat.