A common acronym that teenagers use to stand for "You Only Live Once" usually used in the context of wanting to do something crazy, or fun (usually also stupid). It became popular in 2011 when a new rap song by Drake was released, it was titled "The Motto" it is in the chourus which goes like this
"Now she want a photo
You already know though
You only live once ‒ that’s the motto nigga YOLO
And we 'bout it every day, every day, every day
Like we sittin’ on the bench, nigga we don’t really play
Every day, every day, fuck what anybody say
Can’t see 'em 'cause the money in the way
Real nigga what's up?"
Teenager 1: Aye, you wanna go smoke some weed?
Teenager 2: Nah man i ain't 'bout dat life
Teenager 1: C'mon mah nigga, YOLO
Teenager 2: Nahhhhhh