"Hardcore gaming" is an activity that involves learning every possible maneuver/ability/stage in a videogame not only to win, but to win efficiently, making the fewest mistakes and taking the least amount of time to win. Winning may be collecting items, downing opponents, completing levels etcetera. Winning can be any combination of these goals and may vary depending on the game.
Spending countless hours playing a game may be a result of hardcore gaming, but is not a requirement. Simply playing a game without striving to improve or master the game is not hardcore gaming. That would be casual gaming
After or near mastery of the game, the player can be called a hardcore gamer
for the said game.