Hardcore is a game mode that was introduced to the Diablo franchise in Diablo 2 and has become a feature of the series since it first appeared returning in Diablo 3.
Hardcore in Diablo 4
Hardcore characters are identical to normal/regular ("Softcore") characters in every way but one; when a hardcore character dies, that's it. There is no resurrection, no restarting in town. They're mortal. When a hardcore character dies, all gear and everything in the inventory is lost forever.
To thrive at hardcore it requires dedication and resilience, and successful hardcore players have a different mindset about how to approach the game.
The real die-hard players often select Hardcore as it ups the ante requiring a more cautious play style. This means more casual players often never opt to create a hardcore character.
Hardcore PvP
Hardcore PvP in Diablo 4 will be particularly brutal. If you die as a hardcore character in the game, your character will be lost forever. Taking on PvP as a hardcore character is not for the faint-hearted.