They mean the bags made of uncoated kraft paper, which lets air in very well. "Brown [paper] bag" is essentially the name of this type of bag, which is often us

They mean the bags made of uncoated kraft paper, which lets air in very well. "Brown [paper] bag" is essentially the name of this type of bag, which is often used for school lunches (small bags) or groceries (bigger). Here's an example: https://www.canadiantire.ca/en/pdp/goodtimes-paper-lunch-bags-1-88-l-50-pk-0530200p.0530200.html https://www.canadiantire.ca/en/pdp/goodtimes-paper-lunch-bag... They come in other colors--we had bleached white ones when I was a kid--and those would be fine too. However, it risks confusion with other paper bags, like the ones used in bakeries. These often have some kind of liner (polyethylene, plastic, foil) or wax coating that would impede airflow. The department store paper bags, even if they are pure paper and colored brown, are often multiple layers that would block airflow too.