pH is defined as the negative log of the concentration of hydrogen ions in a solution. (Technically, it is not based on concentration, but activity, however the activity is very close to the concentration for water solutions).
Solutions having a pH that is less than 7 are called "acidic". Solutions having a pH greater than 7 are called "basic". pH equal to 7 is considered to be neutral. This is the pH of pure water at a temperature of 25 °C. Since pH is logarithmic, a change of pH by 1 corresponds to a change in concentration by a factor of 10.