It’s an acronym for material which should be avoided in published material:
no politics, alcohol, religion, sex, narcotics, -isms or pork
(Can’t remember the other ‘s’)
Depending on the country you sell to, your books can be banned if something slips through the net. I once worked on a primary school maths book where we had to brief the designers illustrating a beach scene to draw long-sleeved t-shirts and trousers on the people in the water because bare arms/legs wouldn’t be allowed in certain countries.