There are no sure things in the NFL draft, but quarterback may be the riskiest position of all. No defense revolves around a single player the way every offense revolves around the quarterback. Trust your offense to the wrong young quarterback, and your team isn't going to climb back into playoff contention. Recent history is littered with teams that drafted a quarterback high, only to find out that player couldn't play at the NFL level.
The lessons of history can at least help us figure out how much of a risk each quarterback prospect will be. That's the point of Football Outsiders' Quarterback-Adjusted-Stats-and-Experience (QBASE) projection system.