The acidity and concentration of hydrogen ions.
The pH of a substance tells us the concentration of Hydrogen ions. By subtracting this value by 14, we get the pOH, the same as pH except it's Hydrogen ions.
The higher the concentration, the lower the pH, thus the more acidic the substance.
pH is logarithmic, meaning it has relations to the value of 10 for each step of pH. One-step difference of pH means a difference of 10x the acidity.
For example, if a substance has a pH of 3, then it is more acidic than a substance with a pH of 6. It would have a higher concentration of hydrogen ions, and is 1000x acidic (3-step difference in pH).
You can calculate pH by multiplying the negative log and concentration:
#pH = #- #log[H^+]#
You can calculate the concentration of Hydrogen ions by having 10 to the power of the negative pH.
#[H^+] = 10^(-pH)#
Hope this helps :)