Instagram/Vimeo
The feature seems similar to "tagging" Facebook photos. And it presents a new opportunity for brands to engage users of the social network.
With Photos of You, any Instagram user can add (or, tag) any Instagram account to their photos. An individual can now tag their photos with any other Instagram user, product, or the accounts of businesses.
And a business can do the same.
For example: now, when Nike posts a photo of an athlete to its Instagram account, it can tag the photo so that the athlete can also add the photo to his or her stream. This engages Nike's followers as well as any followers of that athlete, increasing the overall number of views of the photo.
Brands (and anyone, for that matter) can curate the photos
Here's a video showing how Photos of You works. Facebook users will note the similarity to "tagging":