When you mix Acid and Base subtances (better called an Acid-Base reaction) it create a substance that can be used to discover the "Acidity" and "Strongest Base Bonds" in fruits, metals and objects that cannot be analyzed very easily.
FUN FACT!: The first one to provided this concept it was the Chemistry scientific Antonie Lavoisier.
Acids can be find in products that have: Nitric Acids, Hydrochloric Acids, Sulfuric Acids, Perchloric Acids, Hydrobromic Acid, Hydroiodic, etc (Daily Market products like Lemons, Oranges and home cleaners)
Bases, by the other hand, can be found in mostly metals like: Lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, etc (Calcium, Potassium, Cesium, Etc)
Using the pH Box, we can calculate pH of liquid things. Using the central neutal point (Being water) we'll know if the liquid is More "Acidic" than 7 or not.
A buffer solution is more precisely a mix between a strong Acid (For example an "Acid" with a level 12 of "Acidity") and a weak base (For example, a bese with a level 4 of "Acidity") and viceversa. This solution is used to make other susbtance to get more "Neutral" ("Acidity" level 7) and making a substance constant.