It's actually a line from a poem by Shel Silverstein.
"Two boxes met upon the road.
Said one box to the other,
'If you're a box, and I'm a box,
Then you must be my brother.
Our sides are thin, we're caving in,
And we must get no thinner.'
And so two boxes, arm in arm,
Went home to have their dinner."
The meaning is that we are the same on the inside. Our bodies and outside appearances may be different, but we all have similar problems and needs. The only way we can survive is by taking care of each other.