The game's background is basically there's two races, humans and monsters. Monsters got sealed away into the underground, and you are a human child that fell down there. You explore the Underground meeting various interesting monsters with their own distinct personalities.
Every action you make can change the ending in one way or another, and I do mean EVERY action. Even if you restart without saving, the game will remember, and it will be sure to let you know it does. There is every possible outcome for almost every possible option. What I mainly love about this game is that everything is very well thought out. There are plenty of secrets and easter eggs to go back to, making this game highly replayable. Some secrets you have to find by closing and reopening the game.
If I had any complaint about this game, I would say that the beginning is a bit slow, but it's enough to get you immersed in the game. The first character you meet is Toreil that gives you the rundown of the game. You can say, she'll give you a...TuTORIEL?! Yeah this whole game is filled with lame puns.
One main thing I also love about this game are the characters. Each character no matter how nice, mean, annoying, or sadistic they are have a lot of charm and heart put into them. Mind you, if you play through genocide you'll have to eventually kill these characters. Especially if you do pacifist first, it might break your heart to see these characters die, but I think that just adds to the experience.
The battle system is pretty creative too. Basically you're a heart that has to dodge the attacks of various monsters, like a bullet hell game. Sometimes it's simple, most times it's pretty crazy. Although I would say the difficulty overall is pretty fair. There are two options that change the game very drastically. You have a choice of killing an enemy, or sparing it. The more enemies you kill, you gain EXP and gain levels, this path is called Genocide. To reach the True Pacifist ending, you'll have to spare every enemy and boss. Arguably, it's easier to get though the game killing everything, though I would not recommend starting with it. You will have a bad time later on, and it will alter True Pacifist's ending even when you reset.
Overall this game is very well thought out, extremely charming, and a joy to go through even at the tough parts. It's not too long either, for this game only has seven areas. If you're a fan of RPGs like Earthbound, I'd recommend you try this game. Even if you aren't, it's a nice game to start with.