Water also is known as H20 because it has two hydrogen atoms and one oxygen atom for each water molecule. From this base molecule, it can accept an additional hydrogen ion to become H3O, and become acidic. It can also lose a hydrogen ion to become HO, and become basic (alkaline). The pH scale is a measure of whether water has additional hydrogen ions (lower on the scale, acidic), or is missing hydrogen ions (higher on the scale, basic).
Pure water has a pH of 7, and is the center point of the pH scale. It is neither an acid or a base.
Substances that have extra hydrogen ions to donate to the water, are known as acids. If you add an acid to water, it gives the water extra hydrogen ions, and the water becomes more acidic. The more extra hydrogen ions there are, the stronger the acid, and the lower the pH value. This is why if the nutrient solution tests too high of a pH, you can add pH Down to lower pH. The acid in pH Down donates hydrogen ions.
Substances that will soak up and capture hydrogen ions from water are known as bases oralkalines. If you add a base to water, it will take away hydrogen ions from the water, and the water will become more basic. If the pH reading of a nutrient solution is too low, adding a base such as pH Up to capture hydrogen ions and raise your pH.