The second important thing relates currect, resistance, and voltage together. It is called Ohm's Law. Ohm's Law says that the voltage across something is equal to the current times the resistance. That makes sense because if you have a lot of current going through something that has a very high resistance, the charges need a very high voltage to get through. The last imporant thing before we tie everything into your question is the concept of series and parallel.
Series means things are in a line and parallel means they are side by side.
This is important because of the way things in series and parallel work together. Now, back to your question. 2 batteries in series have one special effect, their voltages will add up. So by putting two 1.5 volt batteries in series, you have a total voltage of 3 volts. On the other hand, if you put the batteries in parallel, they do not add up.