An acid-base indicator is a dye or mixture of dye that marks the end-point of the titration. The indicator has a sudden and obvious color change at the pH corresponding to the end-point of the titration. A good indicator must have the following properties:
The logarithmic acid dissociation constant (pKin) determines the pH at which the indicator changes color (end-point).
Read more about various types of acid-base indicators here.