It's a right wing talking point about how some people in the US are being held to account for their actions for more or less the first time, particularly in regard to racism and sexual violence. Cancel culture to the right is wanting to remove someone's status or influence based on past actions. To us, it's just doing the right thing and making people responsible for the distasteful things they've done.

I'm not sure if it's strictly defined. the way I see it used is to describe an (online) trend of "cancelling" somebody for something bad that they have done. the idea is that when somebody is cancelled, they should no longer be treated positively, for example losing their celebrity status.