When I think of tension I think, surface tension. I remember a film about spiders which showed one sitting on top of the water in a glass. The hair on it’s legs created surface tension which kept it from getting wet in the water. It could just sit on the surface, due to surface tension.
I think it’s a great illustration for tension. Any change to the elements involved and the spider would begin to sink, need to swim or grab the edge of the glass to prevent itself from drowning. Tension is like that. The moment before, or the balance between, something else happening. Tension can change your story. As a writer I think you can use tension to develop your plot in ways you hadn’t planned on at the beginning. It brings so many new possibilities and reactions.
Tension is something about to change and that’s exciting.