pH, literally, "pouvoir hydrogeene" or "power of hydrogen" is a measure of the concentration of hydrogen ion in aqueous solution. In precalculator days, the use of logartihmic tables greatly simplified complex calculation, which is why these scales were developed. There should be 2 logarithmic functions on your calculator, log_10 and log_e or ln.
You should be able to tell me the values of log_(10)100, log_(10)1000, and log_(10)0.1; these are all simple whole numbers.