Listening to Venturi on JMV today, he explained "Reaches" I think very well: Theres X# of players who rated Round 1, then the fall off was to Round 3. For teams needing X position, if they dont take those R1 talents, they'll "reach" down to pull them up 1 round where they "should" go due to lack of talent at that position in that particular draft. Teams use this "gamble" as they expect players who they rate higher to drop if theres a lot of those type of players in the draft.
See LB's this year: There were 6 LBs rated in the top 3 rounds, 4 taken in the 1st round. Those 4 Venturi had as R1 talents, but the other 2 were R3. Meanwhile, other positions such as Guards (example only) were in greater supply, likely bringing down several to lower positions where they would be available. So, take the rare talents over the common talent.
Similar to last year's draft, where there was a plethora of Defensive players near the top, while only a few Offensive players. This lead to a rush on Offensive players, which pushed Hooker down to us