Hi i am 15 yrs old an when i was a young girl i was taught 4 diffrent languages an by me getting older?

It kinda messed my mind up .I'm not dum or anything ,but I have a hard time @ time remembering what such an such word means ....but can you please tell me what does the word “that“ means in this sentence ....used to mention something bad “that“ you do not want to affect others .I kno that “that can b used all diffrent kinds of ways ,but I need to kno what that kind “that“ means thanks everybody for your help