Obstruction is the act of a fielder who, while not in possession of the ball and not in the act of fielding the ball or a throw, impedes the progress of any runner. Contact does not have to be made for the umpire to rule obstruction. There are two types of obstruction Type 1 and Type 2.
This informative instructional video provides a clear cut definition of obstruction and shows several plays as examples. MLB Umpire, Ted Barrett, explains the rule. Remember a few things about obstruction: intent is not a factor, contact is not necessary and it occurs when a fielder impedes a runner.