Wikimedia Commons (commons.wikimedia.org) is one of the platforms within the Wikimedia family of projects, which includes Wikipedia, Wikidata, Wikivoyage, MediaWiki (the software on which all these projects run, as does the OpenStreetMap wiki) and other projects. It hosts most of the images, videos, audio and other media files used across Wikimedia projects, and these files can be embedded in other MediaWiki instances, e.g. here.
Wikimedia Commons hosts a lot of photos of more or less noteworthy physical objects everywhere (buildings and all kinds of structures, noteworthy shops, trees, artwork, memorials, signs, parks etc), which may never actually be used in Wikipedias etc, but can hold information valuable to OpenStreetMap mappers (in the image itself or in its metainfo). As such contributing photos to it can also be worthwhile for OSM contributors as well as linking to those photos from OSM objects.
- wikimedia_commons=* - To link OSM features to a Commons media or category