Well, basically Joji, who is a half Japanese, half Australian singer, previously had a YouTube channel which did humour which is usually inappropriate and offensive, humour which isn't meant to be taken seriously (channel is called Filthy Frank, if you're interested).
Anyway, the channel stopped making content in 2017, and he went on to make music.
But on May 2020, some Kpop stans found out about his channel, and they started harassing him on Twitter, calling him a racist and other stuff like that in a attempt to cancel him. They stopped doing that now, from what I've heard