I've seen this symbol pop up a few times, each time claiming to have something to do with Rogue One, but I don't remember seeing it in the film. I tried researching and reverse-image-searching etc. but didn't find much. When is this in the movie, and what is its significance?
It's a "Modified Wren Phoenix crest."
Different colored versions of it appear on some Rebel flight helmets. While it doesn't seem to be associated with any particular squadron, it most frequently appears on Blue Squadron helmets.
The Rogue One Ultimate Visual Dictionary writes that
The personalized iconography worn by pilots on their helmets occasionally identifies their unit origins, homeworlds, or beliefs.
It also gives examples of various pilots who wore the symbol.
[Photos of varying quality incoming.]
It's also available as an insignia option in the official "Rogue One Helmet Creator."
Sabine Wren is a character on Star Wars Rebels who frequently paints this symbol, which the symbol in question is apparently based on.