Acidity (or conversely Basicity or pH) is a measure of the activity of Hydrogen (H) ions in an aqueous solution (solution in which the solvent is water). It is roughly the negative of the logarithm base 10 of the molar concentration (moles per liter) of hydrogen ions. A pH of less than 7 is acidic and more than 7 is basic. pH is popularly thought to range between 0 and 14, but actually there are no limits.
Carbon Dioxide (CO2) in water lowers its pH by reacting with it to form Carbonic Acid which can break down into Bicarbonate or Carbonate, losing protons, thus decreasing pH.