A watering mix applied at transplant time containing beneficial microbes, a wetting agent and optional plant tonics to inoculate the root zone and help moisture spread through fresh soil.
Jeremy's version is RootWise Microbe Complete, Saponaria or Therm-X 70 wetting agent and horticultural aloe in water. He says plain water and good soil are fine but he uses the drench to build the ecosystem he wants rather than leave it to chance.