Obstructing the field is a mode of dismissal handed out to the batsman if in the opinion of the umpire the batsman has committed an act of obstruction, while the ball is in play, so as to prevent the fielder from either taking a catch or effecting a run out or as the case may be.
This again is a rare form of dismissal, which is subject to the discretion of the umpire. Before handing out the verdict the umpire must ascertain where the act was intentional or incidental.
For example, the batsman plays the ball in the vicinity of the pitch and attempts a run. He also finds that the bowler/ fielder is heading towards the ball. In such a scenario if the batsman intentionally tries to block the fielder from fetching the ball or ends up colliding so as to prevent him from gathering the ball, while trying to disguise it as subtly as he can, the umpire may rule him out.