Acid may be added to the “must” (the honey water mixture you’re going to ferment) both to adjust the pH and to balance the sweet flavor of the honey. Yeast prefer an acidic environment. Many other micro-organisms don’t. The acid you add protects the must until the alcohol level creates a hostile environment for the competition.
[There is a discrepancy between older practice in mead making, which tends to advise adding acid at the start, and newer practice, which suggests that it be added after fermentation to avoid problems from too-low pH. This document is in review/revision on this point.]
Acid can be added in many forms. Winemaking suppliers sell acid blends, powder or liquid. Acid is measured in “as tartaric”, or how acidic the must is compared to pure tartaric acid. For example, if the must is 0.5 percent acid as tartaric, it is as acidic as if 0.5 percent of the must were pure tartaric acid. Inexpensive test kits will let you measure the acidity so that you can adjust it. Acid blends are a combination of tartaric, citric, and malic acids. You may be able to get the individual acids used in blends. Each contributes a slightly different taste in addition to acidity. The natural acid in fruits and berries will also acidify the must, for which reason melomels often need no additional acid.