In the instructions the hints and tips section reads as below, but we quite often get asked what exactly we mean by sharpening.
To adjust the taste of your wine you can add citric acid (to give your wine a sharp taste). Most wines will need sharpening. To sharpen the wine add citric acid 1/2 a teaspoon at a time until the desired level of sharpness is achieved.
Sharpening just means adjusting the acidity level using citric acid, which doesn't have much flavour of its own. Often wines which are meant to be drunk cold and have a crisp fruity taste benefit most from the extra acidity, such as white grape wines. For a much more detailed explanations of acidity in wine making see the articles here and here.
It's not possible to easily make your wine less sharp, so proceed with caution when adding the citric acid.