A waterproof bottom is a type of clothing or footwear that has been designed to be resistant to water, preventing it from soaking through and damaging other items in your bag. Full definition
Waterproof Bottom Layer: The bottom layer of the diaper change mat is lined with a medical grade waterproof material. (lilhelper.ca)
It comes with a comfortable cushion and does not take up much space on the bed plus it also functions as a playpen with removable toys and can be used as changing pad with the waterproof bottom. (momwoot.com)
The company makes a larger size in fleece with a waterproof bottom that's 18 x 27 inches and sells for $ 10. (consumerreports.org)