YOLO is a Young People aconymn. It stands for "YOU ONLY LIVE ONCE."
Carpe diem for a new generation, the term ("YO-LOW") is generally reserved for assinine situations. Young people often yell it out as a reason or insurance against repercussions for doing something stupid.
The fad is attributed to Drake (some older young people may remember Drake as Jimmy on the new generation of Degrassi), who uses both the acronymn and the phrase in his song "The Motto." (Purists will point out that the general principle, "You only live once!" has been around longer than Drake has been alive.)
I like to use YOLO when it is least appropriate. "I think I'll take a nap. YOLO!" "I can't go to the bar, I have all of these books to read. YOLO!" I also like to use mundane situations (because napping and reading are undoubtedly awesome) as well. Part of the reason is because I like the rise the hackles of young people.
Old People are encouraged to follow my lead.
"The Motto," Wikipedia
#YOLO: The newest acronym you’ll love to hate, The Washington Post
YOLO: The Evolution Of The Acronym, The Huffington Post