Is when you say you don't like an idol's vocals or dancing or rapping or whatever talent/skill, and someone tells you "can you do better?" or "show us your singing/dancing/etc and let's see if you are any better!"
We are not artists! singing, dancing and rapping are not our job, so why exactly should we be better at them than idols whose job is to sing, dance and rap?
you don't have to be a chef to be qualified to have an opinion of food. It's the same here, you don't have to be a professional music critic to have an opinion on a song or a professional singer to have an opinion on someone's singing. you are not only allowed to criticise art if you can outdo it.