pH is the measure of how acidic or alkaline the aquarium water is on a scale where a pH of 7.0 is neutral and lower is acidic and higher alkaline. This is not to be confused with what is often referred to as alkalinity in marine (salt water) aquariums where this refers to the water hardness measured on the German scale, dkh.
pH is measured on a logarithmic scale so the difference between a pH of 7.0 and a pH of 8.0 is actually a shift of ten times the difference.
Most freshwater aquarium fish are tolerant of a wide range of pH values but some such as Discus are happy at a lower pH of around 6.5 whilst Marine (salt water) fish require a Ph of 8.0 to 8.3 and Rift Valley Cichlids prefer a higher than neutral pH.
pH can also effect plant and algae growth rates, fish spawning and the effectiveness of some aquarium treatments and supplements, but can be controlled to precise parameters with a number of products.