This is a question few tend to answer prior to using the term. By my definition it means a Quarterback that can effectively run your offensive system with minimal errors for around ten years (barring injuries). It doesn't mean the QB has to go to the pro bowl or a Superbowl. Pro Bowl/ Superbowl QBs often are Hall of Fame caliber. Effectively running a system should mean 2-1 TDs to Interceptions and a passer rating of 80+. Using this simple formula you can start to see how difficult it is to find a Franchise QB.
For instance even though the average QB passer rating this season is about 80 only 49 QBs in history have scored this high for a lifetime. John Elway is #50 with a rating of 79.9
But today's NFL is a QB league where passing has become much more important than the old style run first and pass when necessary NFL. The advent of the spread offense in college has taken mediocre football teams to record wins because they don't need the same talent at QB as a pro-style offense. Even though many NFL teams now run 30-50% of their plays from a spread formation there is no such thing as a spread offensive team in the NFL. Why? Because the speed of the end rushers in the NFL leave no time for a QB in a spread to survey the field. Defenses in college are just slower. Some day an offensive coach in the NFL will solve this problem but until they do every team in the NFL needs a QB that can run the pro-style ( 7 step drop & play action pass) offense.
The point of this article isn't to come up with a magic formula for selecting a QB. It is to point out how difficult selecting a QB can be. Taking all of the QBs selected in the first round from 94--06 shows that only about 30% actually become suitable QBs let alone a Franchise QB.. Now the last few years have shown an improvement in NFL first round success. I suspect that is because the cost of these young phenoms is so high that a team is putting more resources into the process. But you still get a JaMarcus Russell Overall #1 bust pick now and again. QBs taken later than round 1 fair far worse. This is why you can't simply take a QB like Pike that is not projected as a first rounder and solve a teams QB problem. At best a second round or later QB will take several years to develop. An example is Drew Brees. He was so slow in developing that San Diego pulled the trigger and drafted Eli Manning ( then traded for Rivers) to replace the struggling Brees.
So, when the experts say that only two or three QBs in this years class are possible first rounders believe them. And realize that probably only one will be a Franchise guy. And if your team needs a QB right now don't plan on taking one in a later round.
Just some random thoughts.