A little hard to explain without pictures but here goes. The secret is to remember that the voltage of a cell is the "potential difference" between two places. In this case the two places are the negative and positive electrodes of one cell. So now if you connect the one cell to a second cell you have a potential difference of say 1.5 v for the first cell and you have another 1.5 v potential difference for the second cell and since the output of one is going to the input of the other they add and the total potential difference is the sum of the two or in this case 3.0 v. Think if they are connected in parallel, then you have the + connected to the + and the - connected to the - of the other cell. When you connect the + of the combined cells and the - of the combined cells to an outside device you are just drawing voltage from the one cell for voltage so you get 1.5 v BUT for current you are drawing from BOTH cells so you can get twice the current but only 1x voltage. In the series connections you get twice the voltage but the current from just one of the cells. Hope this helps.
