Scavenge allows a player to draw resource cards, but only when other cards say that there is stuff to be scavenged, and sometimes cards will tell you specifically what can be scavenged with the "if successful" clause.
For example, Bob moves and flips over a location space that says, "Once per player: There's a huge garden here. You may Scavenge. If successful, take one Herb Resource." Because you can only scavenge when a card tells you to, Bob could only get an herb resource here, not choose to draw a resource card instead. (After all, this is a garden - no word or ore resources here!)
Bob has already missed two rolls this turn by 1, so he decides that he definitely wants to grab some herbs. He calculates modifiers, rolls a d20 and...well, wouldn't you know it? Missed by 1.