Yonko refers to the strongest active pirates in the world of One Piece. Considering that there are only ever four Yonko spots up for grabs at a time, it means only a handful of pirates are ever considered worthy enough of earning the title, since to do so, they need to demonstrate that they possess not only immense raw strength but also powerful Haki abilities. Needless to say, the Yonko are an absurdly strong group of individuals, which is why it's so rare for someone to face off against one of them without being killed in the process.