In MMA, the idea of a divisional no man's land is pretty straightforward.
Basically, it means you can beat everyone in your division except the champion. You lose one fight to that person, you work your way back to a rematch, you come up short in that second one and then you're stuck.
The unwritten rule is two losses disqualify you from a third chance. If the person you've lost to those two times is the champ, your primary recourse is to hope for a different champ.
Junior dos Santos currently finds himself in this predicament, having lost twice to UFC heavyweight champ Cain Velasquez.