For those not in the know:
Leeroy Jenkins is an Internet meme named for a player character created by Ben Schulz in Blizzard Entertainment’s MMORPG, World of Warcraft. The character became popular due to a video of the game that circulated around the Internet. The phenomenon has since spread beyond the boundaries of the gaming community into other online and mainstream media.
The video was released by the World of Warcraft player guild “PALS FOR LIFE”. It features a group of players discussing a detailed battle strategy for the next encounter while one of their party members, Leeroy, is away from his computer. Their plan is ruined when Leeroy returns and, ignorant of the strategy, immediately charges headlong into battle shouting his own name in a stylized battle cry. His companions rush to help, but Leeroy’s actions ruin the meticulous plan, and all of the group members are killed. [Source]