Somnacin is a drug that makes possible both lucid dreaming and dream sharing in the users. While there is brand name version which is strictly controlled, and likely restricted to therapeutic use, there is at least one black market chemist who creates their own modified versions of it.
Yusuf’s Somnacin variant is more potent, stabilising the dreams and also creating the time differential effect between the layers of dream that Cobb’s crew uses in their work.