"Proc Coefficient" (or proc rate) refers to the % chance a skill or ability has to "proc," that is to trigger effect X, usually via a crit. Many of the skills in Diablo III are balanced by tweaking their proc coefficients, and changes in these values in patches are always hotly-debated by fans.
For instance, the Wizard's Critical Mass passive skill has a chance to cut 1 second off of the cooldown time for all of her skills with each critical hit. Thus a proc coefficient of 100% would mean every single critical hit would trigger Critical Mass, while a proc coefficient of 10% would mean that 1/10th of critical hits would trigger Critical Mass. Some skills have more than 100% proc rates and will return double value effects on functions such as Life on Hit.
The proc coefficients are not on Critical Mass in this instance, but on all of the individual attack skills and their run effects. For balance purposes, skills that hit very many enemies many times, such as Energy Twister must have lower proc rates than big damage, high-AP-cost skills that only hit once or a few times, especially on just a few enemies, tend to have considerably higher proc coefficients.
This system is in place on almost every skill and rune effect in the game, and all are balanced differently depending on their utility, how often they hit, how larger is their AoE, etc. Numerous patches have tweaked these values, a process that largely drives which skills and builds are most popular with players.
See the specific wiki articles with proc coefficients for every skill and rune effect in the game.