Any romantic or sexual interaction between female characters.
Moreover, it's tagged as MariAri and MariAri Day on Pixiv.
What Pixiv tagged is usually irrelevant to Danbooru. Pixiv makes heavy use of joke tags, for example and since they don't use pools, they also tag very subjective things.
Case in point, this picture doesn't show any kind of romantic interaction between girls, so it's not yuri.