AHH omg okay, I'll try my best to explain in a way that you can understand ;w;
Okay so, theres this human, thats you, and you fall down into a place in a mountain where the monsters were banished after war with the humans. This little flower dude tries to kill you, but Toriel saves you. She walks you through a few easy puzzles before you're on your own for a bit, and blah blah blah.
This is where the game really starts to show it's charm...actually.....I can't really explain it at all, even after attempting to for the 3rd time today p.p
I suggest at least watching the first playthrough of it, Cryaotic does it well, and it's easier to watch than to explain anyday ;w;