What is pH
This is a subject my high school teacher quickly covered as:
"It is a logarithmic representation of the concentration of H+ ions
The pH scale measures how acidic or basic a substance is
pH<7 alkaline [basic]
This didnt make sense at the time, a magic concentration of H+ ions [pH7] is neutral but any fewer or any more and it is not neutral, but the reason why this occurs s quite a hard concept to understand, if you want to understand it see reference 
Why is pH important in Hydroponics
The availability of chemicals to the roots is heavily dependant on pH[see figure below]. In short, get the pH major wrong and your plant wont be able to use the expensive nutrients in the growing fluid you just made.
[Nutrient availability vs pH, thicker lines is better]
If you have ill plants, pH is a hard one to find as the issue because it will usually appear to be a deficiency of P. making you believe that you need more P [Phosphorous], when in fact you just need to change the PH to make more P available to the plant.
What is the "Best" pH
This is a hard one to pinpoint, it depends on the types of plants in the system and has to accommodate any other living organisms in the system [fish].
It is commonly quoted that a pH of 5.4 to 6.4 is the best range for most plants 
That is why we will have a set point of 5.9 for our pH controllers in many hydroponics/aquaponics.
Please do check that your fish are happy with this pH and consider researching the optimum pH for the plants in your system.
 Bodie V. Pennis " Essential pH Management in Greenhouse Crops: pH and Plant Nutrition" B 1256
A Publication of the International Plant Nutrition Institute (IPNI) "SOIL PH AND THE AVAILABILITY OF PLANT NUTRIENTS"