It's the idea that, basically, if we each have our own paths to walk, so to speak, and if they are all unique to each individual person, then that must mean every possible permutation of a "life" is being felt by someone, somewhere.
For any one person to live a life of (for example) good fortune and health, one person somewhere must also be experiencing a life of the opposite. That if I'm lucky enough not to deal with loss at a young age, someone else must have. Or on a smaller scale, that if I have a good day, there is also someone having a bad day.
It's not quite a personal "Yin and Yang" sort of balance of opposites; more an objective coexistence of every possible experience. I'm trying to find a term or concept to call this, since I'm sure I got it from somewhere or someone smarter than me has come up with it before.