What makes something fast fashion.

I'm very curious about fast fashion as I've recently been more involved in protecting the environment. I live in a small city and the only stores we have here are big fast fashion corporations and one value village. I try to thrift the majority of my clothes but it's not always easy. I'm 15 and so my mom buys my clothes for me, I usually insist we go to value village but recently shes been wanting me to buy clothes from urban behaviour. I personally hate spending my parents money but I'm not allowed a job so I always agree because its her money not mine. When we do go to the fast fashion clothing stores, I always hang onto the clothes for usally 2 years and try to repair them when I can as well as donate them. So I'm wondering if its really fast fashion if I hold onto the clothes and donate them. I know the clothing is also bad for the environment but I don't really understand how and it would be great if someone could explain :)