Lactose is a milk-based sugar that is not fully fermentable by beer yeast. (Not to be confused with Lactic acid or Lactobacillus.) Generally recommended to add near the end of your boil. Adds body and residual sweetness to sweet stouts, balances harsher bitter notes in some brews. Do your research and experiment with varying amounts. Lactose has contributed to some amazing stouts out there, with flavors all along the spectrum of chocolate milk, java mocha, coffee, cookies n cream, you name it.