Undertale is an RPG developed by Toby Fox. You play as frisk, a nondescript human child that has fallen into the world of monsters. The game focuses around the morality of how players usually aproach RPG games and the choices they make in them, possibly to a comedic extent. The monsters in this game are humanized as having lives, families, hopes and dreams. This is intended to be a change to what players are used to in this genre, where monsters are usually mindless enimies that attack the player on sight.
The players experience revlolves around frisk and their adventure through the underground. The player makes choices that change what ending they get. Choices could be what you say to characters or what you choose to do. But the only choices that really change what ending you get is who you decide to kill and who to spare. The player explores the Underground. The underground is the home of the monsters ever since the humans sealed them under there hundreds of years ago.
Playing like all the monsters are real people and trying your best to help them as opposed to hurting them gets you the "Pasifist" ending. In this one friendships are made and the monsters are freed from the underground and onto the surface. This ending is concidered to be the true ending among fans.
Killing anything at all will ruin your pacifist run and could turn it into a neurtal ending. The neutral ending comes in many forms as what you do in game effects the phone call you get at the end of the game informing you about what happened in the underground after you left, some of these endings can be good.
Playing the game like you would a normal RPG, grinding areas for enimies, killing every enemy for EXP and gold and trying to rush through the entire game awards you the "Genocide" ending. The genocide ending is by far the worst that you can get. Frisk becomes a phycopath murderer that kills all in their way. Regular bosses are tougher, they are fighting for their peoples survival rather than just to capture you this time.