The term "over titration" is not one I am familiar with. I suspect it's another name for 'back titration' however. In back titration, you want to determine the concentration of some substance A. However, for whatever reason, direct titration doesn't really work. So what you do is measure out a known amount of some substance B that will react in a known stoichoimetric manner with A molecules. You put in enough B such that you react off all the A and there is some B left; that is, you put in excess B. Then you do a regular titration to measure B, using some third substance C and an indicator.
Once you know how much B you have left after reacting away all the A, you subtract that amount from the amount of B you started with. This tells you how much B you used up getting rid of all the A. Then you use stoichiometry to calculate how much A there was in the first place.
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