Karl Fischer Titration simple explanation
Attempting an almost non-chemical description, you need to know that the concentration of a sample, $c$, equates the number of moles $n$ per unit of volume $V$; expressed as $c = n/V$.
For once, a known volume $V$ of your liquid to be analyzed for its concentration $c$ of water into is injected into a chamber equipped with a stirrer and electrodes.  There is a stock of reagent solution for which you know ahead of the analysis its concentration. You equally know ahead how many molecules of the reagent will react with how many water molecules to yield a balanced reaction equation (stoichiometry).  The reagent solution shall react with water only.
The addition of the reagent solution to the analyte is monitored; for one, you know the volume of the reagent solution added.  Because of this known volume added, and the known concentration of the stock of reagent solution, you know how many molecules of the reagent were added.  The mixing and stirring ensures that each molecule of the reagent quickly encounters a molecule of water to react with.
For two, during the addition of the reagent solution to the analyte, the electric potential between the electrodes dipped into the analyte is monitored. Initially, the addition of a little volume of reagent solution changes this potential only a little.  However, continuing the addition of small volumes of reagent solution, there is one point where this change of potential suddenly is large.  This is the point when 1) basically all the water molecules reacted with the reagent, 2) the electrodes now monitor an excess of the reagent in respect of the water molecules, and 3) the titration may be stopped.
Because you know the volume of the reagent (of known concentration) added and the balanced reaction equations used in this analysis, you may calculate the number $n$ of water molecules consumed to reach this point of sudden change of the potential.  However, because you know both the number of water molecules consumed as well as the volume $V$ of analyte initially injected, you may compute the concentration of water molecules in the analyte, too ($c = n/V$).

The Karl-Fischer analysis offers a rapid characterization of organic liquids (minute scale), provided they do not react differently with the reagent.  Samples containing alcohol, for example, may differ in the underlying reaction equations and stoichiometries from samples containing water.  Generally speaking, to work well, typical samples submitted do not contain more than 1 mol/L of water (about 18 g/L).