Undertale Routes are the paths of the game that a player can choose to go down, the "Neutral Route" is the typical playthrough, the player would have to kill and spare all kinds of monsters in order to see this route's ending, the "Pacifist Route" is the usual choice of playthroughs, the player would have to spare every single monster they come across, random encounters to bosses, certain side quests become available in this route to show the monster's bonds with other monsters or even the player, this route has a different ending compared to the Neutral Route. Finally, the "Genocide Route" is another popular route to choose, but due to its difficulty, it's clear why this path is typically avoided by casual players, the player would have to kill every monster in the game, random encounters and bosses. As the player goes through, the monsters aknowledge the player's actions and try to stop you from killing everyone.