When the road is blocked ahead of you, that blockage is an obstacle that keeps you from getting where you want to go.
The Latin word that is the source of the word obstacle combines parts that mean "to stand in the way of," so an obstacle is something that stands against what you want to do. In a dramatic piece, the obstacle is the person or thing that keeps a character from achieving his or her goal. This creates the conflict in a play: since conflict is what drama is all about, the obstacle is the source of all drama.
