Titration involves determining the concentration of a chemical solution by reacting it with another chemical solution of known concentration.  If the volumes of each solution are carefully measured and the reaction is allowed to go just to completion without an excess of one reagent added, then the volume of the known solution added can be used to calculate the concentration of the unknown solution.

An example is adding a solution of NaOH of unknown concentration via a buret to a solution of HCl of known concentration in a beaker.  If the addition is done using phenolphthalein as the color indicator to reach the equivalence point, then the moles of NaOH added are equal to the moles of HCl in the solution to start with.  Since you now know the number of moles of NaOH added and also the volume of the solution added, you can calculate the concentration of the NaOH solution.