To put it simply it is a measure of how basic or acidic the water is. 7.0 is neutral. Most water in this part of the country is strongly basic; (8.5 - 9.2). Adjusting the pH in a pond is not strictly necessary for the health of the fish or plants but if you bring the pH down to slightly acidic (pH of 6.5 - 7.0), it dramatically reduces the growth rate of all types of algae. If you can keep the pond clean without adjusting pH that's fine, however if you maintain the pH in that range it will make a MAJOR difference in algae control.
There are many test kits on the market, some liquid and some strips. It may seem easier to use test strips but they are less accurate and more expensive than the liquid tests. The two I like the most are "Wide Range pH Test Kit" -- PondCare and "pH Test Kit" -- API. I prefer the wide range because it accurately reads above 7.6, but either work well.
In most cases you will only need to reduce the pH. For this we recommend our own brand of pH down. It is formulated specifically for mid-west water (starting pH of 8.5 to 9.0). There are many brands out there but with most you need to add well over the recommended dose when dealing with St. Louis water.