"pH"

pH is a measure of acidity or alkalinity, and the acronym stands for "power/potential of Hydrogen", i.e., the number of Hydrogen ions in a given mass or volume.

For simplicity pH is often said to range from
0 (maximum acidity) thru 7 (neutral), to 14 (maximum alkalinity)
(although there are actually some acids and alkalies which exceed these numbers).

pH is a "logarithmic" scale, a scale where you count orders of magnitude (or the number of zeros at the end of the number). An example will clarify this:

If you have three solutions, of pH 8, 9, and 10 respectively, the solution with pH 9 is 10X as alkaline as the one with pH 8, and the pH 10 solution is 100X as alkaline as the pH 8.

Similarly if you have solutions with pH 5, 4, and 3 respectively, the solution with pH 4 is 10X as acidic as the one with pH 5, and the pH3 solution is 100X as acidic as the pH 5.