It's a game about... choice. Is that helpful? No?
Undertale is pretty unique. I don't want to tell you too much about it. It's best at:
- Surprising you with mechanics
- Surprising you with emotions
- Surprising you with meta-awareness
- Surprising you with humor
- Just plain surprising you
The overall game art style looks very retro in screenshots, to the point of looking amateur - especially when it comes to the terrain. But, when you get into the game, you see that the work put into animations, dialogue timing, sound effects, and character art is tremendous. There is a LOT of charm lurking underneath the surface of this game.
If you played Earthbound on the SNES, you'll find a lot is familiar. An even bigger emphasis is put on character relationships and non-combat solutions. There's a kind of... twisted innocence, I think is what I'd call it, that it shares with its spiritual predecessor as well - a sense of the world's tremendous capacity for evil, and wrongness, and at the same time just being silly, cracking jokes, and having fun.
The combat is just bonkers. The acronym I would coin to describe it is Contextual Shape-Based Bullet Hell (CSBBH). It really is a brilliant approach, feeding conversational elements into the gameplay, taking something very basic and making it feel novel, something that changes drastically with every foe or friend you meet.
It's also a game that, from a storytelling perspective, got me to feel invested enough that I experienced some real anger at some of the "surprises" - so angry I uninstalled it. Then, I re-installed it, and played through from the beginning again. And I don't regret that. I'm completely satisfied with my experience. I can't say that of many games now, especially not having the time to play as much as I used to. I really felt like I had to play it again.
You can get through the game in 5-6 hours, though it offers much more with subsequent playthroughs, and might involve a significant time investment if you want to see all it has to offer. I really enjoyed the writing, the charm, and the world the game builds. I highly recommend it.
If you're still not sold and want to learn a bit more, I suggest the Rock Paper Shotgun article on the game. It manages to avoid spoilers while giving you a little more on the details of what makes it unique.