In this animation we have ignored the autoprotolysis of water:
2H2O(l) <---> H3O+(aq) + OH-(aq)
Kw = 1.0 x 10-14
Experiment shows that in pure water at 25o, [H3O+] = [OH-] = 1.0 x 10-7 M. This is very small compared to the concentration of water molecules which is about 55 M. However, because Kw is an equilibrum constant, no matter what we add to water, the value of Kw must be preserved. Therefore, if the concentration of H3O+ is increased by adding acid, the concentration of OH- must decrease and if OH- is increased, H3O+ must decrease.