Do yall seriously not have google wtf why are you asking me
Trans people are people who are assigned one gender at birth, but do not identify with it, and change it to their preffered gender, be that socially or physically.
Cis people are people who identify with their assigned gender.
THE POINT IS that until very recently, there was ZERO representation of Transgender or Queer characters in media, and if there was it was played off as a joke. Undertale is a game that does a good job of respectfully representing us and giving us real, well-rounded characters. That is why people care.