pH is the acidity or alkalinity of the nutrient solution. It is a measurement of activity of dissolved hydrogen ions. They are most active in the zone where all the elements remain in solution and available for plant uptake.
Plants can survive in the pH range 4.0 to 8.0. Below 4 there is a danger of the roots being burnt and some minerals are not available to plants. Above 8.0 some of the minerals can be precipitated or are not available to the plants. If roots are ever exposed to extremely low or high pH, turn off irrigation, bleed 50% of the tank, add fresh water, get pH spot on and then turn irrigation back on. Most times you can save a crop with this method.
The most important thing to remember is to keep pH between 5.5 and 6.5. Aim for 6. All the elements are available in that range. When plants are growing in good light and warm conditions, the normal trend is for the pH to rise and we have to add a pH lower (acid solution). In cool, dark, short day conditions, it can be normal for the pH level to fall and we have to raise the pH with pH raise (alkali solution). As a rule, as plants feed, their root waste (sometimes in the form of ethylene gas) is basic and raises pH. In media based systems, microbes eat most of this up so pH is fairly stable.