Dramatic tension is literally the tension or suspense of the narrative. This usually takes the form of a 'puzzle' or 'riddle' that must be solved. The most well-known dramatic tension that most people can pick out is the main tension. This is the most prevalent and noticeable disturbance or problem that brings about a need to solve a question throughout the story that is usually introduced near the beginning of the story and is solved near the end. Main tensions are basically the recognizable conflict situations that cause the reader to ask "What is this problem?" "How is the character going to solve it?" "What is the cause?" etc.
Smaller tensions also exist and are experienced through scenes or chapters that keep the reader engaged and wanting to find out more and sometimes take the form of sub-plots or smaller narratives that have their own questions to be solved and problems to face, no matter how seemingly insignificant.
To sum up dramatic tension, it is a problem or problems that arise in the narrative that sets the pace and tone of the plot keeping the reader interested and engaged. Without dramatic tension the narrative will fail because the reader/viewer will lose interest as they will not be questioning or taking part in the overall story. The point of any narrative is to make the viewer/reader feel like they are a important part of the overall solution of any story (Through solving the dramatic tension) and this is what dramatic tension is all about.
English Literature, Drama and film studies at varsity :)