To cancel someone (usually a celebrity or other well-known figure) means to stop giving support to that person. The act of canceling could entail boycotting an actor's movies or no longer reading or promoting a writer's works.
Used to be a good way to protest against someone online, but it's devolved into what Twitter virgins do when they actively look for and inevitably find something semi-racist someone said 17 years ago because they hate the person.
They know that people can change over that long of a time period, but they do it anyway because they're bored and don't have anything productive to do.
(12 years ago) popular person: haha you've been norway n*gga penis'd
twitter virgin: I hate this guy what if I found something racist in his tweet history?
twitter virgin: huh he said "dumb" but that's not clickbait enough to cancel him
twitter virgin: OMG HE SAID THE N WORD TIME TO SPAM THIS ONLINE AND CANCEL HIM
other twitter virgin: heh i'm bored lemme see what's online
other twitter virgin: HOLY SHIT (popular person) SAID THE N WORD IN A MEME 12 YEARS AGO TIME TO MURDER HIS FAMILY LMFAOOOOOOO #popularpersonisoverparty
cancelled or "cancelled culture" basically means if you or anyone does something thats considered "bad" (being racist,sexist,manipulative etc..) you would most likely be "cancelled" , basically means no one would look at you in a good way anymore, no respect, etc. this is mostly found on the internet and people who are mainly "cancelled" are celebrites and online influencers
famous person: i fucking hate gay people , rape is good and we should build the fucking wall! 95% of the internet after you said that: "CANCELLED!"