This food grade lactic acid (88% solution) is used for artificially adding a sour flavor to your beer without having to use bacteria that can contaminate your equipment. It is also used to lower the pH of your brewing water if it is too alkaline for the yeast to thrive.
After 1 week of fermentation, add the ½ oz Lactic Acid. Give a gentle stir using a sanitized utensil without disturbing the sediment (boil utensil in water for a few minutes to sanitize). Quickly replace lid and continue fermentation.