Tension is the state of being under stress, or more specifically in this case when the Earth’s crust is under stress.
Tension often occurs along continental plates and is a strong force which drives plates apart under the ocean as well. Where ever tension occurs, the crust begins to crumble and weaken, often causing fault lines, almost cracks in the crust.
Generally tension occurs by pulling two parts of the crust apart, resulting in an area of lower elevation, possibly even a valley. IF you can imagine this, you can think of tension almost like pulling apart silly putty, or gum. When you pull at both ends, the middle sections become stretched out and weak, often breaking if stretched too far. Tension affects the crust in the same way.