Think of dividing some continuous material into super small cubes. If you pick three perpendicular surfaces of these cubes you can define 3 stress tensor components for each, that represent the pull/push/sideways stress per distance unit that this cube element feels under any conditions. The combined stress of each of these cubes describe the stress that the material feels at every point.
They are useful because in isotropic mediums under simple stress you can find relatively simple expressions for the stress tensor in every point. <a href=" https://essayschief.com ">Click here...</a>