The game is called Undertale because it is a tale of a human who falls into the underground, a place where monsters have been sealed away long ago as a result of losing a human / monster war.
It is an adaptive storyline game with 3 different storylines, there is Pacifist, where you befriend the monsters, Neutral, where I think you Ignore the monsters and focus on getting back to the surface (I'm not sure because I haven't played the game either), and Genocide, where you kill every monster that crosses your path.