This is something that we learn in school, but something that can be easily forgotten about. Electrical circuits and how they work.
Essentially, electrical circuits are very easy to understand. All you need is a power source, an output source, and wires connecting them. If you are missing any piece, your electrical circuit will not work. As long as you have your power source and wires connecting this to your output product it will work. When there is a break in the circuit your product will not work.
Most circuits will also have switches involved. When these are included you have to have your switch turned on in order for the circuit to work. When the switch is turned off, there is a break in the circuit meaning the electricity can not flow through it freely.
In order for your electrical circuit to work perfectly, you need to have a perfectly flowing circuit with no breaks.