I hate to be that guy but if you don't know that you shouldn't be wiring things up in your house in series you really shouldn't be wiring things up unless you're following directions and double checking yourself every step of the way. I'm actually kind of bamboozled as to how he could have managed to do that based on the image he posted. That fixture should have been wired up in parallel internally and all he should have needed to do was to hook up the live and neutral leads to the proper loose wires on the top of the fixture. Even if he got it backwards it still should have worked fine. Unless he built that fixture himself, or if it was specifically designed to undervolt incandescent bulbs to achieve a target color temperature (which would work fine but it'd be easier and more versatile to use a dimmer switch since you could crank them up to full brightness at will), it should have just worked. You really have to go out of your way to wire something up in series as household electrical stuff is pretty much as idiot proofed as it can be
I'm not an electrician or anything but for a very basic run-down of series vs parallel, if you wire up a set of devices, lightbulbs in this case, in series, you're splitting the voltage between them. If you were to wire them in parallel, as intended, you're splitting your amperage instead.
Anything that plugs into your house is going to be designed to run on 110-120 or 208-240v, usually tolerating anything between those two or even a bit lower, and pulling anywhere from a fraction of an amp up to 12-13 for standard 120v plugs (or 15 amps for brief durations). As you'll notice, you've got pretty narrow ranges on the voltages, but quite a wide range on amperage that you can play with. Very simple devices, incandescent bulbs for example, will work in series just fine, they just won't be as bright. You don't want to run an AC motor on decreased voltage though, that's really bad. It will instead increase the amps that it's pulling in an attempt to hit its intended speed, and amperage is the primary driver of waste/lost heat, so the motor will very quickly burn itself out. Never try to run a fan on any kind dimmer unless it specifically mentions being designed to do so, is what I'm saying. You can get PWM dimmers now which theoretically should work but it's still not the best idea
Where series circuits tend to be more useful is when you're working with batteries. Rather than dividing your voltage like with a consuming device (bulb, blender, etc) you can multiply it to hit a specific power target. If you have a 20v power tool, as an example, rather than a 20v cell, you'll have 5 3.7v batteries in series (and potentially multiple 5x3.7v series in parallel for larger packs, they're nominally 20v because that's what they have at full charge but that drops almost immediately as you start using it), giving you approximately 18v at 10-30 amps for most of the pack's runtime
