Titration Curves
Below is a graph depicting the changes that occur in the pH of a solution during a titration of a typical strong acid with a typical strong base.

At the start, before any NaOH has been added, the solution is simply 25.00 mL of 0.20 M HCl. Since the HCl is a strong acid and is 100% dissociated, the initial pH is quite low.
As we add more and more NaOH solution we are of course affecting the concentration because of volume changes. Between 0 and approximately 24 mL of NaOH added, a large increase in the pH is not seen. All that is happening is that the added base is changing some of the acid to the salt, NaCl. NaCl, neither of which ions hydrolyses, so the concentration of remaining acid gradually becomes more and more dilute. By adding just enough base to neutralize the acid the pH should change drastically to a pH of 7.00 or neutrality. The addition of a little more base will cause the resulting solution to become quite caustic.
The graph above indicated that a rapid change in pH with a small change in volume of standard solution is desirable if the indicator is to give a sharp colour change at the endpoint.