pH stands for power of hydrogen, which is a measurement of the hydrogen ion concentration. The pH scale measures how acidic or basic a substance is. The total pH scale ranges from 1 to 14, with 7 considered to be neutral. A pH less than 7 is said to be acidic and pH greater than 7 is basic or alkaline.
Certain foods increase the acidity of the mouth, i.e. they drop the pH level. This includes sugary drinks, snacks, and some grains. The lower pH of the mouth creates an optimal environment for bad bacteria, which eventually causes tooth decay.