If you connect the cells in series the voltage drop across the 2 cells will be the sum of the individual voltages.
If you had 2 cells of 1.5 V then you would create a 3 V battery as shown in (a):
If you look at (b) the 2 cells have been collected in parallel. The voltage drop across the battery is now only 1.5 V but the capacity is greater.
In (c) the 2 cells have been connected back to front. The voltage drop is zero and the resistor will draw no current.
There is not much use for this arrangement though a potentiometer uses this idea if you wish to measure the e.m.f of a cell without drawing any current from it.