"TKO" stands for technical knockout in UFC, according to HowStuffWorks. A technical knockout is one of the ways to win a UFC fight. The official UFC website notes that fights end in decisions, submissions or knockouts.
HowStuffWorks notes that there is no standing eight count in UFC, unlike in boxing. Technical knockouts, therefore, can be called regardless of whether a fighter is standing. A technical knockout occurs when the referee halts the fight after finding one of the fighters unable to defend himself. This differs from submission, in which scenario a fighter submits either physically or verbally to his opponent.