What is stress? 

Stress is basically the internal resistance of an object to a force applied to a unit area. And that's the scale of the tensor. A tensor also has magnitude, direction, and a plane with respect to which the stress is defined. Basically it is not the same at all points. It changes angle and direction. Basically the force creating surface tension can act at any angle and direction. The normal stress is the component perpendicular to the plane of the surface and the shear stress is the component perpendicular to the plane of the surface. And it can be either compressive or tensile. Compression is positive normal stress and tension is negative normal stress. Commonly Stress is used for solid materials and is expressed in MPa/GPa. It has no measuring device but it can be measured.