pH is a measure of how acidic or alkaline the nutrient solution is. The pH scale runs from 0 to 14. 7 being neutral, below 7 is classed as acidic and above 7 classed as alkaline. The majority of plants prefer it to be mildly acidic when growing hydroponically, therefore somewhere between 5.8 and 6.5, as otherwise nutrient availability can be affected and some elements become unavailable to the plant.
Most tap water is between pH 7 and 8. Monitoring the pH is vital, as it helps to ensure that the plant has the optimum amount of nutrient mix available to it that it needs. As you can see from the chart, some elements are less available to the plant, when the pH is out of the optimum range. Please note that adding nutrient alters the pH of the overall solution.