Tension, also called Dramatic tension, refers to how keeping an audience hooked to a given story or play. Or in other words, it refers to the audience’s involvement regarding the play.
In order to create and maintain such dramatic tension writers often make their audiences make assumptions instead of giving them the entire information. In this sense, the phrase "fatal vision" might lead an audience to make assumptions regarding what such vision could mean, which might or might not be correct. Therefore, they would remain involved in the play in order to see if their assumptions are right.
The main theme of "The Road Not Taken" is that life is full of choices which will define our destinies. The speaker spends a while deliberating when he comes to a fork in the road, which symbolizes a choice he must make in his life.
