That's your soffit. Might be a simple "push it back in place", but that soffit is an important part of your roof, so you might want to have someone who does this, take a look inside, just to make sure that everything still looks OK. Had a neighbor a few years ago, with a similar soffit fault. He found out that there was a leak (not visible from the outside) that was starting to rot some of the rafters in an area like that.
In your case-- might be nothing, just close it up. But, IMHO, not something that you want to put off too long, now that you have seen it.
Don't "bang on your iphone", use it to call a roofer. 😎