What is a membrane?

What is this mysterious thing? membrane, which is used in footwear and clothing (marked with the tex label)?

The membrane is a thin layer that is placed between the upper part of the shoe and the lining to prevent moisture from entering the shoe. The membrane has 1,4 billion microscopic pores per square meter. These pores are about 20 times smaller than water droplets, so water cannot pass through them into the shoe.

However, water vapor molecules (sweat) are smaller and therefore escape through the pores of the shoe. However, water vapor molecules can only escape if the temperature inside the shoe is 15 degrees higher than outsideThis is also the reason why summer shoes usually don't have a membrane.
So shoes with a membrane = waterproof shoes.

However, it is worth remembering that shoes with a membrane are not as waterproof as rubber boots. Shoes with a membrane can be used on wet grass or sand, in light rain and snow. However, if there is heavy rain and sleet, we recommend using rubber boots.

Shoes with a membrane should not be dried on a radiator or in a drying cabinet, as this may damage the membrane. To dry the shoes, we recommend removing the insoles, placing newspapers inside and keeping the shoes upside down until they are dry.

It is from here, which waterproof barefoot shoes to choose for spring-autumn.

Information comes from the page https://exploreborders.com/