Google Photos is ground zero for the company's crazy machine learning technology. At Google I/O 2017, a number of new features were announced for Photos, including suggested sharing based on who's in a certain photo. But for that to work, Google needs to figure out who is who, and now the Photos web app is asking users to identify what they look like.
Upon visiting photos.google.com, you may be welcomed by the above screen. Google already uses machine learning wizardry to match faces to people, so all you have to do is select which face is yours. Once you do that, you'll be able to search for "me" in the Photos search to find pictures of yourself. According to a recent teardown, this option should appear at some point in the Photos app as well, but it didn't for me.
Presumably, this will be far more useful once the other features announced at Google I/O show up. Let us know in the comments if you have received the pop-up.