pH is defined in the aquatic literature in several ways; most people accept that its name 'pH' is an abbreviation for pondus hydrogenii, or weight of hydrogen and leave it at that. Without delving into the more obscure (to most of us!) technical aspects involving such things as positive or negative ions, let's settle for knowing that it is the yardstick by which the acidity and alkalinity of water is measured.
Ranging from 0 - extremely acidic - to 14 - extremely alkaline, the range of pH readings that interest the aquarist lies roughly between 6 and 8.5, a small portion of the whole range and centred around the so-called 'neutral point' (neither acid nor alkali) of pH7.
Freshwater can be from acidic pH5 of Amazonian jungle forest streams with much decaying vegetation to at least pH9 of the tropical soda lakes of Africa. Domestic tapwater, depending on where you live, often ranges from pH6.6 to around pH8.
Normal seawater, and most artificial saltwater mixes, come out around pH8.3 at time of mixing although with increasing time of usage in the aquarium this will tend to fall, indicating that a partial water change (with new salt mix) is due.
Always take pH readings at the same time of day (it doesn't matter what time, of course!) so that you get a fair comparison between readings. During the day, the aquarium plants will remove any carbon dioxide from the water so giving a higher pH reading; overnight, the plants will give off carbon dioxide thus a morning reading will reveal a lower (more acidic) pH value.
Other factors causing pH changes include:
Testing pH involves mixing re-agents with a sample of aquarium water and using a colour comparison chart to find the pH value. At the top end of the scale, there are electronic meters that can be used to do the same thing which is far more convenient if you have many tanks to test.
The most important thing to bear in mind is that the scale used is logarhithmic - each whole number on the scale represents a change of 10 times from that of its neighbour. This means that water of pH7 is ten times more alkaline than water of pH6 but also ten times more acidic than that of water of pH8.
Attempting to adjust pH is a delicate business and should be undertaken with the utmost caution. Only deal with very small changes at a time and let the fish get used to them before adjusting further.
Finally, it may be that the exact pH value is often quite unimportant with only the rate of change between readings being vital to understand.