“”If you talk about the pH of the "body," you're either an altie or have access to a very large blender.
|—The You just might be an altie if... list|
In chemistry, pH is a measure of the acidity or alkalinity of a given substance (solution). As a most basic explanation, it's an inverse log scale of the concentration of positively charged hydrogen ions, so very acidic solutions are pH 1, while the more basic (alkaline) solutions are higher numbers. There are numerous caveats to what "concentration of hydrogen ions" actually means in practice, however, so refer to Wikipedia or a chemistry textbook for all the gory details.
At RationalWiki, pH woo is the catch-all term for all forms of woo that hold that the cause for (some) diseases are changes in "the body's pH balance" - the body (or the blood) being "too acidic" or "too alkaline". It already sounds like hearkening back to the days of the four humors, only with two.
There is no one pH within the body, so the idea of the body's pH being balanced between "too high" and "too low" is on unsteady ground. Even if you were to localise it to, for example, the pH level within the blood alters quite naturally depending on levels of dissolved carbon dioxide and depending on diet and physical activity. Even then, that's just talking about blood itself, the balance within individual cells can differ considerably too.
Like many things that alternative medicine and body woo focuses on, there are natural coping mechanisms to deal with things like incorrect pH. Only in very extreme cases would you ever need to do something drastic about it.