This is an optical device which is the ancestor of modern cameras. From the 17th century onwards some artists used it as an aid to plotting compositions.
Essentially the camera obscura consisted of a lens attached to an aperture on the side of a darkened tent or box. Light reflected from the chosen subject outside of the box passed through the lens and was projected on to a surface on a much smaller scale inside the encased area. The subject could then be traced.
This mechanical means of recording images is known to have been employed by Canaletto. The Delft artists Fabritius and Vermeer may also have experimented with it.
