In an acid / base titration, the acid & base react until one completely neutralizes the other. Say I'm adding -OH to HCl. Once all the HCl is used up, the pH spikes up because the -OH is in excess. Knowing the concentration of -OH, and vol added, we can figure out some stuff about the acid.
Similarly, in a redox reaction one thing is oxidized and one thing is reduced. Once the neutralization point is exceeded, excess titrant builds up. Knowing the volume of titrant added to reach that point and concentration helps to figure out some stuff about the molecule in question.