Picking Poison
"Read option" is not a precisely defined term. In general, the term usually refers to plays where the quarterback and halfback run in opposite directions, and a defender to the side of the quarterback's run path is left unblocked as the option key:
As with all option plays, the offense is making a bargain with the defense: the offense gets to leave a defender unblocked, letting the quarterback occupy a defender with an option read who would otherwise have to be blocked or occupied with a fake. The flipside of this arrangement is that the defense can dictate who carries the ball. To defend an option play, the defense designates at least one defender for each potential ballcarrier (note this is a necessary but not sufficient condition for defending the play - many TV commentators suggest that the option can be stopped just by ensuring a defender is assigned to each of the dive, keep, and pitch, but there are still plenty of ways for an option play to succeed even if every defender knows his assignment, not least of which is that the defenders with those assignments might get blocked). The defense also arranges these assignments to force the ball to the runner they perceive as least dangerous, and to funnel this runner to an area where they have more defenders, starting a cat and mouse game of adjustments and counter-adjustments between the two teams.