Ceramides are lipids (fat) which holds the barrier together to help prevent moisture loss and protect skin from the external environment. A compromised barrier system is one which does not contain as many ceramides, cholesterol, and free fatty acids as compared to a healthy one. This then leads to skin dehydration, roughness, wrinkles, sensitivity, and makes skin more prone to irritation.
There is a lot of confusion surrounding dry skin and dehydrated skin. After all, they sound like the same thing right? While the terms dry skin and dehydrated skin are often used interchangeably and they can exhibit some of the same symptoms, they're actually quite different.
Basically, dry skin is caused by a lack of natural substances that keep skin healthy, while dehydrated skin lacks water and moisture in the top layer of skin.