Pretend you're an electron. You're sitting at the negative terminal of a battery, but the voltage of the battery makes you WANT to flow to the positive terminal, any way you can.
There are a bunch of things connected between the terminals, and they're all DC conductors. Like resistors, light bulbs, motors, electromagnets, coils. So you know that there's a path you can follow to get to the positive terminal.
You start out. You flow through one component or device at a time, making your way closer to the positive terminal all the time.
If you EVER come to a place where there are two or more components connected together, and they both can take you in the right direction so you have to choose which one to use, then at that point you're entering a PARALLEL part of the journey.
As long as there's never more than one component to choose after you finish the one you're in now, you're traveling through SERIES parts of the trip.
The whole trip could actually have some of both kinds of parts in it. Then, technically, it would be called a "series/parallel circuit". There would be choices to make at some junctions, but not at others.
