pH is a measurement of the acidity or alkalinity of a solution. A solution is acidic if the pH is between 0 and 6.9 and it is alkaline if it is between 7.1 and 14. The ideal pH for most hydroponic nutrient solutions is between 5.8 and 6.2, with some exceptions.
The pH of a nutrient solution is one of the most important factors for the uptake rate of essential nutrient ions. Maintaining your pH within the correct range will give you plants access to all the essential ions for optimal growth.
Nutrient lockout will occur if pH levels are outside optimal levels.