When a structure is put under some load, some parts of the structure are put under more stress than others. Stress tends to concentrate in certain parts of a structure. If these stresses are not properly analysed, the structure mail fail in an unexpected way, which may be disastrous.
To better predict failure, and perhaps find some way to prevent it, engineers make models of structures out of transparent plastic. When the model is viewed through a polaroid, stress concentrations show up as concentrated bands of different coloured light.
