Hydromel was a vaccine used by the Vanir to stave off Lazar's disease. Hydromel was a phosphorescent green organic compound. It was inserted in the Vanir's armour to protect them from infection from the disease. They needed to constantly replenish the Hydromel.
The Vanir's employers used the Hydromel in order to keep them working at Terminus, since they would cut off their supplies if they stopped obeying. (DW: Terminus)