The Horizon is awfully painful to read. It gets you constantly feeling lost, fearful, shocked and nauseous while reading it. This webtoon is scary.
It's a post apocalyptic story. Some criticized how weak the worldbuilding is and how it terribly lacks context. But that is done on purpose and in fact, adding some context would actually ruin the story. It proves that's it's not an excuse for lazy writing but that it's meant to be that way.

Worldbuilding choices and impacts.

The world of The Horizon is a huge huge mess. Some sort of event apparently reduced humanity's population dramatically and destroyed most of civilization. Yet, there's still war, rampage and chaos. We don't know why it happened, how it happened nor where the story is being told. For this story in particular there's no need for some geopolitical explanations.
The universe lacks context and that's exactly why it's so scary and good, it contributes not only to the story setting but to the substance. What matter isn't the consistency of the worlbuilding but the feelings conveyed through the narration. The drawings are the representation of that. The background are always half erased, and blurry. It makes you feel lost and dizzy and you can't even locate where the story take place. It's nowhere particularly, so you can say, it's the same everywhere. In this way, we can concentrate on what's really important : the characters and their feelings.
We follow the story of The Boy and The Girl who meet each other by coincidence and they both have the objective of walking down the road until it's end. You can think of this story as a tale. The characters are named by their roles/parts of their identity and the thing that is really important is the message. This story is a tale, that is, as far as how the story is told goes. What the story actually tells is nothing fabulous nor magical.

Despair and the horror of the absurd

As previously said, you will probably have trouble reading this webtoon because it is VIOLENT. It's harshly and utterly violent. And we're not talking about graphical violence. There's some disgusting scenes but it isn't gore at all. This story is morally and psychologically violent.
It is true horror and despair. It the horror of the absurd and it's even scarier that it doesn't need blood nor fights to make it scary for it's readers.
It's not graphic violence but moral/psychological violence. Random shit happens for no reason to the characters and they can't do anything about it. The worst is that no one knows why all of his happens. The characters live in constant fear and they have to go through traumatic experiences and there's not even a reason why.
The terrible things happening are so random and absurd that you start to wonder why the characters even keep on living. They wonder that too. Dying would be way easier.
Yet, the only reason they keep walking on the road is because it's the only thing to do, it's the only thing they have left and ironically the only they have control over even if it's absurd.
Due to the narration being told through the eyes of children, we get to a lot of naive reasoning such as "why would we kill each other let's all be friends and chill". Out of context, it is so idealistic that it sounds dumb and cheesy but in that world where nothing make sense and where everything is fucked up to the very core this line is actually incredibly wise and this type of reasoning is the only thing that actually made sense since the beginning of the webtoon.

More things to say

Both characters go through different developments as the story progress. In this cruel world The Girl desperately wants to avoid seeing anything negative while the boy has more and more issues to find anything positive at all. And that's the big question, is there truly anything positive? The road is more or less a metaphor for hope and life. In 20 chapters, this webtoon can convince you that life makes absolutely no fucking sense, yet you're supposed to keep hope? To keep moving forward, walking down the road, just like The Boy anf The Girl? Is there hope at the end of the horizon? And that's where it hurts because the principle of the horizon is that you can't reach it.
And when you think about it, this shit's kinda meta (no) because it's exactly like us readers. For those who already read it, there wasn't any real reason to keep reading after some chapters. It's a webtoon full of shocking, sad, awful and disgusting moments. Yet we finished it, why? Because we had the hope that at the end of the scroll something good was going to happen to the kids.

cOnClusIon

This isn't a regular webtoon this is an absolute masterpiece. Everything serves to the substance from the drawing to the scrolling format and the webtoon is just everything it is trying to be, it just works absolutely well. It is flawless. The most incredible thing is that despite all the absolute horror and the cruel absurdity, this is still a positive webtoon. It doesn't lie to you by saying that in the grand schemes of things, there was a reason you were born or by saying that you will always be happy or that everything will work out for you if you have a positive mindset while still giving an inspiring message. It is one hell of a beautiful, deep, tragic, human and tearjerker story. It's one of the best thing I've ever read and I hope you will experience it too, striving for the end of the horizon!