Rule 40(b) was a new rule introduced by the 2012 Rules Committee stating that no candidate can be considered by voters if they don't have the support of a majority of delegates from at least 8 states. This was put in place by the "establishment" to hold off Ron Paul's small but zealous group of supporters.
In 2016, before the convention starts, the Rules Committee will have the ability to remove the rule (though by default, all rules stay the same). Without removing it, potential candidates like Kasich, Ryan, Rubio, Romney, etc, won't be able to win.
Cruz/Trump and their delegates have a strong incentive to prevent it from changing (both should qualify with more than 8 states). Everyone else (including the "establishment") HAS to get it changed to win. This will be a scrappy battle based on how many supporters from each group get put into the Rules Committee by the various states (each state appoints 2 people).
A few good articles about it:
More speculation that rules changes are coming, including changes to Rule 40(b):