A common practice in many multiplayer video games. Depending on the game type it can have different meanings/ramifications.
In an RTS
(real time strategy) one usually yolorushes by sending his entire army to neutralize a certain target or objective, often leaving his own base extremely vulnerable. The goal of the maneuver is to catch the opponent off guard who usually wont expect such a stupid and risky move. The attacker does so hoping to win the game at that point or probably die trying, hence making it truly YOLO
In an MMORPG
yolorushing usually involves a member of a team who decides to rush the boss by himself before his teammates can support him for whatever reason. Recklessly rushing certain parts of a puzzle or dungeon also applies, usually to the detriment of the team and possibly the death of the "yolorusher".
In an FPS
one commonly does a yolorush by (unadvisedly) separating from his team in an attempt to get kills or simply hog all the glory in the match. Such an action could be spurred by other motives such as rage at a certain player. Either way, the yolorusher either gets lucky and ends up looking like a fucking stud or ends up being a stain on the carpet thanks to some sniper or a well organized team.
Player5: fuck it! Ima yolorush blue, some1 cover my ass or I might get pwnd. *player5 proceeds to emptying his base of all units and blitzes blue's base*