Like Freshmen 15, you supposedly will gain 15lbs in your freshmen year. Cubicle 15 is the same, the phenomenon of gaining 15lbs in your first year working in a cubicle.
A: How's your new job?
B: Damn dude, all I do is sit. Cubicle 15, man.
A: I'm getting fat ever since I start that 9-5 life.
B: ikr, #cubicle15.
It means that life working a 9-5 job in an office cubicle, hence the term "cube" derived from cubicle.
A: Dude, my back hurts from sitting all day at work.
D: Yeah, that cube life.
Some Millennial: I'm not with that cube life. / I ain't about that cube life.
Senior undergrad: I'm not ready for that cube life yet. I want to travel and see the world.
A veteran (senior office worker) in that cube life: The cube life can be soul-crushing and soon you will just be zombies tip tapping on your keyboard.