Alkalinity is the total carbonate hardness. Or a reading of KH or dKH (degrees carbonate hardness). This is many different minerals that are alkaline(basic). This does not tell you how acidic or alkaline(basic) water is because it does not take into account the acidic things that neutralize the alkaline(basic) minerals. PH does tell you how acidic or alkaline(basic) the water is, but does not let you know why it is acidic or alkaline(basic) .
When we use the term pH we are talking about the amount of hydrogen ions, or whether it is acidic, neutral, or basic, from 0-14. Acidic is less than 7, Basic is more than 7. Older book will say alkaline instead of basic and this is how things get confusing.
The term Alkalinity is used to mean different things depending on how it is defined (It can mean basic or carbonate hardness). When we use Alk we mean the total amount of carbonate hardness. Carbonate hardness is an old term and I should say something like “The buffering substances like Carbonate, Bicarbonate, borate, hydroxide, silicate, and phosphate.” But who has the time for all that. These minerals resist the change in pH. This is why we commonly call them buffers. The proportions and amounts of these minerals affect the pH. In simple terms if you don’t have enough alkaline minerals in the correct proportions meaning the Alk is low, the pH will be low. If you are able to keep the Alk up where it should be, does not mean the pH will be where you want it to be, because there are acids that also affect pH.