EC stands for Electrical Conductivity. It is a measure of how well a material (in the case of hydroponics, a solution) conducts electricity. This is a function of the amount of electrolytes (charge carrying particles) in the solution. More the electrolytes higher the conductivity.
pH range from 0 to 14. The pH of hydroponic solution controls the availability of the nutrients to the plant. A change in the pH can change the form and therefore reduce or increase the availability as different nutrients are available at different pH level.
In simple terms EC Provides number of nutrients in nutrient solution and pH provide its availability to plants.