A Strain Gauge is a device that measures the deformation of an object. The most common type of strain gauge consists of a flexible insulating backing, which supports an etched metal foil grid of known resistance. The strain gauge is bonded to the test piece using a suitable adhesive.
When the test piece is deformed by stressing it, then the strain gauge also becomes deformed and its resistance changes. The amount of resistance change is linearly related to the underlying strain, which is in turn related to the stress.