Brake bosses are the interface on the frame & fork that hold brake calipers. I have worked on bikes for decades without looking at them. They are like an invisible part, they are there but no one sees them.

Recently I was removing a brake caliper and I removed the brake boss. It was very strange, I never knew that was possible. It turns out the bosses screw into the frame. Therefore if there is not enough bonding compound they can unscrew. You can just screw them back into place, however you may want to use some glue, so they stay put.