pH, short for potential hydrogen, measures the acidity or basicity of a brewing fluid such as the starting water (liquor), mash, wort, or beer. A fluid with a pH less than 7 is acidic, and a fluid with a pH greater than 7 is basic or alkaline. If a fluid has a pH of 7, such as pure water, then it is considered neutral.
pH is a very important factor in all grain brewing, and different geographical areas and water sources can have dramatically different pH levels and mineral contents that impact the pH of a mash. Additionally, the style of beer that you brew will have a significant impact on your mash pH level. Typically the darker the color of the malts that comprise your grain bill, the more acidic your mash will be. This is important because the enzymatic conversion of starches to sugars only effectively occurs in a mash that has a pH between 5.0 and 6.0; ideally your mash would be between 5.2 and 5.5 pH, which is considered optimal.
A great starting place for dialing in your mash pH is to get a copy of your municipal water report to learn about the pH and mineral composition of your brewing water. Software such as BeerSmith has great tools built in that will help you determine when brewing salt and mineral additions are required to help you bring your pH in line when comparing your local water to that of a specific style of beer. Lastly, purchasing some inexpensive pH testing strips is an effective way to ensure that your mash pH is at an appropriate level.
A wide selection of home brewing pH testing equipment can be found here:
Photo of ColorpHast pH testing strips used during the mashing phase of a home brewed beer.