I couch it all in the language of leds. LED's have a specific number of volts required to get them to light.
most take about 3.4V but since reds require 2.4v to light up this brings in resistance.
This brings in wiring circuits in series or in parallel. Series increases voltage, parallel increases available amps. Knowing when to use them is the trick.
This brings up the idea of why and how to we used battery packs and why do you in fact need multiple batteries and are they in fact in series or parallel and why.