I think they mean to buy some water jugs, or fill some empty jugs with water so that you have water on hand for water changes for your fish in case there is a shortage or your water gets turned off. I used to live on a well and we did this because if the power went out, so did our water pump and we would be without water. So we filled up empty milk jugs with water and kept them in a cabinet for when this happened, so we had water to flush the toilet and drinking water for our pets, etc.
ETA: If your house floods or you have to evacuate for any reason, you could probably put your fish in cups temporarily. You could go to the pet store and by those little plastic "critter carriers" or whatever they are called, or even a small aquarium, something like this perhaps?