Minecraft Hardcore mode is when you have a Survival world and you have to complete all the achievements without dying even once(but you can use Totem of Undying (How do you put that in plural?) ). It is difficult, but worth it, I would say, for bragging rights. DanTDM has his own Hardcore Survival series. He is the 1st person I know of that has a Hardcore Survival series. Here is the 1st video in that series.
And, as of April 28, 2020, here is the latest episode of his Hardcore series.