Simplify the following text for a Grade 10 native English-speaking person.
Original: A shear stress , denoted ( Greek : tau ) , is defined as a stress which is applied parallel or tangential to a face of a material , as opposed to a normal stress which is applied perpendicularly .
Simplified: Shear stress is a stress state where the stress is parallel to the surface of the material , as opposed to normal stress when the stress is vertical to the surface .