It's actually quite a complex game, with moral decisions, interaction with others, and many other complex ideas.
In Undertale, you are a human that has fallen into the Underground, where "monsters" (I use the term loosely because it carries a negative connotation, which, as you find out quickly in the game, does not apply to these characters. The term is simply used to give you a biased opinion of what to think about these characters, and then it deliberately proves them wrong in order to make a statement about stereotyping and generalizing) have been sealed off for centuries after a war with the humans.
There are three basic ways to play through Undertale: The Neutral run, the True Pacifist run, and the Genocide run. The different ways have sub-endings, and each way affects how the game ends. See, in the game, you are given the option to FIGHT or to ACT. ACTing includes talking to the characters or interacting in basically nonviolent ways. The end goal there is to SPARE them.
If you go through the game sparing everybody, meaning not fighting anyone, and you befriend all the characters you can, you will get the True Pacifist ending, which is considered by most to be the best ending. It's the stereotypical (but not really) "Happy Ever After."
An interesting note about the True Pacifist ending; after you reboot the game after completion, a character pops up and warns you:
"There is one last thing. One last threat. One being with the power to erase EVERYTHING... Everything everyone's worked so hard for...you know who I'm talking about, don't you? That's right. I'm talking about YOU."
(Spoilers for the Genocide run ahead)
Now, if you go through the game FIGHTing everyone, you gain EXP. The more EXP you gain, you gain LOVE (LV). As you go through the game in this way, you will notice that all the characters begin to hate you. Towns that you enter will be deserted and when you speak to the NPCs, they will more than likely say hateful things. At the end of the game, after you've killed everyone, you find out that EXP is an abbreviation for Execution Points. The more Execution Points, the higher your LOVE, or Level of Violence. Throughout this run, you are constantly referred to as a monster, worse than the characters around you; you are so evil, you are viewed as inhuman. It's a dark eye opener to how much damage you've truly caused in the game. Another interesting thing is that after you've completed the genocide run, it alters how you play the game in other runthroughs after.
The three types of playthroughs and how they affect endings as well as other runs are easy to equate to the real world:
- You can go through life neutrally, making a mix of good and bad, ethical and unethical decisions and actions, and you will be okay. Nothing major is going to happen to you; you'd end up in a purgatory of sorts, if we thought about the consequences as an afterlife.
- You can go through life trying to be the best person you possibly can be, making good decisions and being kind to everyone; this is obviously the more encouraged path, and thought by most to be the most rewarding. The aftermath would be heaven.
- Or, you can go through life being evil and making unethical decisions and being selfish and careless of those around you. This would lead you to hell.
You could also compare this game to real life in that every choice you make has a consequence. If you make a bad decision, the game remembers. Even if you reset, it remembers the choice you had made and will call you out on resetting. In life, every conversation you have with someone, every choice you make, will alter something. It will effect something. There is no such thing as "It doesn't matter."
If you make poor choices in life, you will most likely end up unhappy; versus, if you make good choices, you will most likely end up happy.
There is a character named Flowey (the one who gives that speech written earlier). He's sadistic and cruel and cold and uncaring. At the end of the game, one of the characters warns, "Be careful out there. There are a lot of Floweys in the world."
I think this game is so important because it makes such thinly veiled allusions to the real world; it has a moral, a message, for the player to understand. It is so important to draw attention to these things, it is so important to inspire the target audience (teens and young adults) to make good choices and be good people.
It made characters like Flowey and the Genocide run in its entirety in order to warn you not to be that way. You get through it and you truly realize that you are the villain in the game. From those that I know who have completed it, it made them feel so awful, so they tried to play Pacifist again; but the ending was altered. They could not go back to how it was. It lets you regret being that way in-game so you don't have to in real life.