It's the second most common picture men take (after actual dick pics). It's when you nestle your smartphone in your crotch, point the camera up at your face and snap a picture.
It's popularity is due to two factors:
1) Millinials are required to chronical their lives on social media, which includes a visual timeline of their aging faces.
2) Culturally, it's less acceptable for men to take pictures of themselves than women, so they're forced to hide this act.
These pictures are usually taken from the driver's seat at a stoplight, or at a group dinner of some sort from below the table. If you're at the receiving end of a rdp, it's a huge honor, so make sure to send one back.
Dick: Hey, did you get Tom's rdp snap?
Harry: What's that?
Dick: A reverse dick pic? Also known as a reverse crotch shot, it's when you put your phone between your legs and take a pic of your face. It's not a flattering pic so it means something when you receive one from a bro.
Harry: I guess me and Tom aren't close like that...