Rookies making an immediate impact can certainly happen, but it's obviously more rare for a late-round draft choice. Many times, these players are just games that NFL teams take late in any given draft. This is probably the case with the Indianapolis Colts and Tim Smith.
Smith was a sixth-round choice out of Alabama and has NFL size at 6'4" and 320 pounds, but he didn't supply a lot of quarterback pressure from his interior line spot in college, and that probably isn't going to suddenly make a jump in the pros.
Instead, Smith would have to show in training camp and preseason games that he can use his size and strength to stuff the run.