Allowing your eyes to wonder. Looking at women.
Used as military slang. LIbO is short for "Liberty." Liberty is an official term for time off in the military (similar to leave). When one's eyes are supposed to be straight ahead in formation, looking around is called "putting your eyeballs on liberty." Drill instructors or unit leaders will often become very upset when this happens.
Also used when not in formation as a joke or colloquialism.
OOHHHHHH, GOOD TO GO THERE RECRUIT!!! YOU JUST WANT TO BE ON EYEBALL LIBO. You'll pay.
Hey man, lets go to the beach for some Eyball LibO.