When someone is talking about acidity in coffee, they are referring to the range of flavors the coffee offers and not pH levels. Acidity consists of citric, malic, or tartaric acids the bean expresses when you brew it. The quality of acidity shows up in the flavors you taste. Lighter or Single-origin roasts are more likely to express smoother flavors, while darker roasts are more robust and often mentioned as brighter.
How coffee is roasted can change the perception of that bean’s acidity. Darker roasts can show caramelized sugars, as lighter roasts show more fruity or juicy flavors.
If you want to see the difference in acidity (pH-wise), try putting a cup of soda, orange juice, wine, and water next to each other. This will make it easy to tell how coffee’s pH level of acid is lower than most beverages you consume daily.