Newbie question, what is a nerf?

i'm very new to the forums so i hope this doesn't sound too stupid but i'm very unfamiliar with the lingo around here.
what does nerf mean? from where i'm seeing i assume it means something about making a character easy to get but i'm not sure.
i basically trying to figure out if i don't complete the emperor event a rough estimate of how long i might have to wait. i'm waiting on 15 shards for STH to be at 4 stars so i now i should end up with 100 shards. but i'm not sure i'll make the 5 tier in time.
people are talking about the dev's nerfing him after the event. my absolute favorite character from star wars. i want him.