Desiccation is a state of extreme dryness and severe dehydration. This serves a weakness to vampires, as without any digested blood in their system, their body begins to severely dehydrate and dry out.
For vampires to sustain themselves, they need to consume human and animal blood to survive. Without having consumed blood in their system, they will start to become extremely weak and will dehydrate entirely, which results in their bodies mummifying. Desiccation will not kill vampires, but will leave them trapped inside a mummified body like a corpse, unable to move or recover unless they feed on blood.