* larger, green circles represent Wikidata items with an image

* larger, red circles represent Wikidata items without an image

* smaller, blue circles represent Commons images

* smaller, yellow circles represent Wikipedia articles, in the current language edition

EDIT: After reading the documentation page [0] linked in another comment, I see that there is legend in the "layers" popup on the upper-right. It did not at all occur to me to look in this location; I would have guessed that this icon let me change the underlying map tiles (e.g. transit, traffic, topography, etc).

0: https://meta.wikimedia.org/wiki/WikiShootMe