Impasto is paint applied thickly enough for ridges and marks to remain visible. The paint is doing two jobs: creating an image and forming a physical surface. Light can catch raised marks differently as the viewing angle changes.
This differs from painting an illusion of texture on a smooth surface. An artist might suggest rough fabric with fine marks or build real ridges into the paint itself. Looking for that distinction changes how you read a reproduction and what you seek in the original.