The 5/10 rule is an important no-limit concept that first appeared in Bob Ciaffone & Stuart Reuben's excellent book, Pot-Limit & No-Limit Poker. To quote directly from the book:
"When contemplating calling a raise because your position is good, you have a clear call if the raise is less than 5% of your stack, and a clear fold if it is more than 10%. In between those numbers, use your judgement."
It's a good rule for calling a preflop raise with a pocket pair in hopes of hitting a set. The driving force behind the concept is the implied odds in a given situation. If you get your set, but the opponent only has 5BBs after the initial raise, calling to hit the set in the first place is incorrect.